Even though Marvel Studios has released numerous trailers and teasers for Avengers: Endgame, they've kept several important details about the highly-anticipated film a secret to keep fans at the edge of their seats upon its release. We know that the first four phases of Marvel Cinematic Universe films have been building up to this epic showdown against Thanos and his all-powerful Infinity Gauntlet, but viewers are still in the dark about how Earth's Mightiest Heroes will be able to defeat the Mad Titan and which formidable adversaries or difficulties they'll face during their mission to save the countless lives Thanos took away with a snap of his fingers in Infinity War. 

Marvel is so serious about avoiding spoilers and plot leaks for Endgame that they even denied Spider-Man actor Tom Holland access to the film's full script, and he wasn't even told who he'd be acting opposite of in advance. The studio isn't taking any chances this time around, but by scouring the Internet for casting information, pictures from the set of the film, and interviews with MCU stars, and by looking really closely at the film's promotional material, it's still possible to piece together some important details about Endgame. 

Viewers who just can't wait until the film's April 26 release to know who the Avengers will be facing off against in this year's ultimate high-stakes battle are in luck, because we've pieced together all of Endgame's potential antagonists. Here are 12 Villains Rumored To Appear In Endgame (And 8 Who Are Confirmed).

RUMORED: HELA

Hela Marching Down The Bifrost - Thor Ragnarok

The Marvel Cinematic Universe version of Thanos had a misguided yet fairly noble reason for assembling the Infinity Gauntlet and wiping out half of all life across the galaxy. He stood by as his home planet of Titan collapsed due to overpopulation and a lack of adequate resources and wanted to ensure other planets wouldn't suffer in a similar fashion. Thanos decided that the only way to bring balance was to snap his fingers and eliminate fifty percent of existence.

In the comics, his motivation was much different. There, Thanos wanted to snap away countless innocent lives simply to impress his crush, Mistress Demise. Ever since Hela was introduced in Thor: Ragnarok, fans have assumed that the Goddess of Demise will act as the film replacement for Thanos' beloved lady, and think she may reappear in Endgame. 

CONFIRMED: THANOS

Avengers Infinity War - Thanos in armor

The first several trailers and teasers for Endgame made it clear that the remaining Avengers plan on mounting a counter-attack against Thanos to avenge their fallen teammates, loved ones, and the rest of the innocent lives taken away by the Mad Titan's snap. But for some reason, the promotional material didn't actually offer glimpses of Thanos himself.

That changed with the release of the film's final trailer, which came out on April 2, the day fans finally became capable of buying their Endgame movie tickets. In the "first look" trailer, Thanos appears before Earth's Mightiest wearing his battle armor from Infinity War, taunting them for not being able to deal with their failure.

RUMORED: THE SKRULLS

Talos Skrull in Captain Marvel

The MCU is starting to take serious strides in exploring life outside of Earth, and this year's Captain Marvel introduced viewers to the Skrulls, the shape-shifting aliens who have created all sorts of problems in the Marvel comics. The film initially depicted the Skrulls as villains but went on to explain that their primary focus was simply on finding a home for their race.

It's definitely possible that there'll be truth to the rumors that Carol Danvers will bring a shape-shifter or two along with her in the Avengers' fight against Thanos, in an attempt to trick the Mad Titan. He may wipe out Captain America or Iron Man and think he's won, only to discover that he simply eliminated Skrulls who were willing to sacrifice themselves for the good of the universe. Or, they could work for Thanos disguised as the Avengers' fallen friends to mess with their emotions.

RUMORED: THUNDERBOLT ROSS

Captain America: Civil War Trailer 2 - Thunderbolt Ross

Those who were wise enough to stick around after the credits of Avengers: Endgame know that Nick Fury and Maria Hill were both among the casualties of Thanos' snap. We don't, however, know the fate of another one of the Avengers' supervisors, General Thaddeus "Thunderbolt" Ross.

Ross was a villain in The Incredible Hulk, and his strict enforcement of the Sokovia Accords in Captain America: Civil War created a divide among Earth's Mightiest. He doesn't exactly have the best relationship with several of the remaining Avengers, but they may need his and the government's help if they want to take down Thanos.

CONFIRMED: LOKI

Loki in The Avengers with Scepter

Loki lost his life at the very start of Infinity War while trying to defy and eliminate Thanos, but that likely won't be the last we'll see of the beloved trickster. It's highly unlikely he figured out a way to survive his termination, but leaked images from the set of Endgame make it pretty clear that he'll pop up in time travel or flashback sequences.

Set images show the heroes who fought in the Battle of New York from Avengers all together and wearing the costumes they wore during that iconic moment in MCU history. We're not sure what will make these characters relive that war against Loki and his Chitauri army, but if Endgame explores that epic fight, it's likely a villainous past version of Loki will reappear as well.

RUMORED: THE GRANDMASTER

It hasn't been made clear yet in the MCU whether or not The Grandmaster from Ragnarok and The Collector have any sort of connection with each other, but in the comics, the two powerful cosmic beings are brothers. Thanos ended The Collector's life off-camera in Infinity War to procure the Reality Stone he was given by the Asgardians in the end credits of Thor: The Dark World, and some fans think the Grandmaster will try to avenge his sibling in the next film.

Jeff Goldblum's wacky character was an instant fan-favorite, and since he has an assortment of battle-ready warriors from his Contest of Champions at his disposal, he could actually be a big help to the Avengers... as long as he doesn't try to capture all of them once they're tired from fighting Thanos.

RUMORED: THE COLLECTOR

The Collector in the MCU

The Collector has yet to truly establish himself as a fearsome villain in the films in quite the same way as he's done in the comics, but the MCU version of the character has always made it clear that he's clever, crafty, and willing to do whatever it takes to survive and succeed. Since he had the Reality Stone at his disposal before Thanos added it to his Gauntlet, it's possible that The Collector found some way to alter reality and survive Thanos' attack, and that he's simply waiting for the Avengers and Thanos to take each other out so he can reemerge and reign supreme.

As one of the powerful Elders of the Universe, it seemed strange that he was simply and seemingly easily eliminated off-screen.  Fans shouldn't be surprised at all if the rumors of his return turn out to be true.

CONFIRMED: NEBULA

Guardians of the Galaxy Vol 2 - Karen Gillan as Nebula

Before Nebula and Gamora finally just stood face-to-face and acknowledged that their childhood rivalry was entirely Thanos' fault, Nebula was a major villain the MCU. Luckily, the two "sisters" forgave each other in Guardians of the Galaxy Vol. 2, and Nebula made it her mission to eliminate the man who ruined her life and pinned Gamora against her.

She joined some of the heroes on Titan in battling Thanos in an attempt to avenge Gamora and was the only person other than Tony Stark on that planet to survive Thanos' snap. The trailers have shown that Nebula will join the Avengers in trying to take her adopted father out, but that doesn't mean she'll solely be a protagonist in the film. In the comic book version of Infinity War, she donned the Gauntlet and became just as big of a threat as Thanos had been while wearing it.

RUMORED: RONAN THE ACCUSER

Ronan With His Cosmic-Rod - Guardians Of The Galaxy

With the leaked set photos of the Avengers in their old costumes making fans speculate that the heroes will be traveling through time via the Quantum Realm to change the fates of the Infinity Stones, it's definitely possible that they'll visit the Battle of Xandar in an attempt to claim the Power Stone from Ronan the Accuser.

Viewers loved Ronan's surprise appearance in Captain Marvel, and they'd probably enjoy another look at Star-Lord's infamous Guardians of the Galaxy dance battle against him

CONFIRMED: CHITAURI

Since set photos reveal that Ant-Man will supply Captain America, Iron Man, and Thor with some sort of device which will let them go back to the Battle of New York, we know we will also see the return of the Chitauri. These menacing alien warriors came dangerously close to helping Loki take over the planet in Avengers, and even though our heroes have matured and grown stronger since then, they still shouldn't take the Chitauri lightly.

The Chitauri are an incredibly powerful alien race, and unlike most villains, they are entirely impossible to reason with or talk to. If they join forces with Thanos, he'll be even more unstoppable.

RUMORED: EBONY MAW

Ebony Maw shushing Thor in Avengers Infinity War

Now that Thanos has all of the Infinity Stones inside of his Gauntlet, he is capable of doing pretty much anything and everything imaginable. Thanks to the Time Stone and the Reality Stone, he could easily bring back any of the other villains from the MCU's decade-long history to aid him in his fight against the remaining Avengers, but there's one antagonist fans suspect he'll revive for sure.

Ebony Maw didn't survive for much of Infinity War, but in the short time he appeared in the film, he established himself as an incredibly formidable foe. The Maw had magic on the same level as the Sorcerer Supreme's and seemed to be the Mad Titan's most trusted ally. It'd be surprising if Thanos didn't enable his return.

RUMORED: JUSTIN HAMMER

Several of the Marvel Cinematic Universe's most memorable villains were eliminated in the same film they were introduced and will likely never be seen again. Justin Hammer, however, managed to survive Iron Man 2 and has simply been sitting in a prison ever since, waiting for his chance to return and try to best his rival Tony Stark.

Hammer wasn't nearly as skilled or intelligent as Tony, but he was still a capable weapons manufacturer and helped build War Machine's armor. With Tony still stranded on Titan, some fans are speculating that the Avengers may have to turn to Hammer for help in creating weapons powerful enough to take down Thanos.

CONFIRMED: CROSSBONES

Crossbones looks ahead.

Fans assumed they'd seen the last of Crossbones after the HYDRA soldier sacrificed himself in an attempt to take out Captain America in Civil War, but actor Frank Grillo recently revealed that the villain will be making one more appearance in Endgame. 

During a UFC Unfiltered podcast interview, Grillo shocked listeners by stating, "He makes an appearance in the next Avengers movie but it's a flashback." Brock Rumlow never interacted with any of the Infinity Stones, so his addition to the film is a definite surprise, but it's possible that by stopping his Civil War explosion, the Avengers could prevent the creation of the Sokovia Accords and be more united in their battle against Thanos should they return to the war in Wakanda from Infinity War and try to stop the snap from ever happening.

RUMORED: HYDRA

The biggest fan theory circulating the Internet about Endgame is that the Avengers will be using the Quantum Realm to go back in time to moments where they encountered Infinity Stones, in an attempt to create their own time-displaced Gauntlet or to prevent Thanos from ever assembling his. This mission could potentially force them to cross paths once more with HYDRA, as the villainous organization briefly possessed Loki's staff and the Mind Stone which was secured inside of it.

With HYDRA loyalist Crossbones confirmed to appear in the film, it's definitely likely that at least a few HYDRA agents will pop up to complicate our heroes' journey, even if this fan theory turns out to be inaccurate.

CONFIRMED: RED SKULL

Red Skull in Avengers Infinity War

The Avengers' failure in stopping Thanos from wiping out half of all life across the universe was definitely the biggest shock of Infinity War, but the reveal that the Red Skull has been living on Vormir and acting as the guardian of the Soul Stone ever since he was launched into space in Captain America: The First Avenger was still exceptionally surprising.

The Red Skull was stranded on the deserted planet for what felt like a millennia and bound to protect Vormir's Infinity Stone, but when Thanos took the Stone, he apparently earned his freedom. In a Q&A on Twitter, the Russo brothers stated, “Red Skull is free to leave Vormir, and he is also free to pursue his desire for an Infinity Stone."

RUMORED: PROXIMA MIDNIGHT

Ebony Maw should definitely be the first Black Order member Thanos brings back to life with his Infinity Gauntlet should he decide to call in some reinforcements in Avengers: Endgame, but Proxima Midnight wouldn't be a bad second option. She held her own while fighting Okoye, Black Widow, and Scarlet Witch all at once, and her leadership in Wakanda was instrumental in helping Thanos and his forces win the day.

With so much power at his disposal now, Thanos technically doesn't need any help defeating the remaining heroes, but it'd be entertaining to see the Avengers have to contend with Proxima again.

CONFIRMED: OUTRIDERS

The Avengers and warriors of Wakanda successfully took out countless Outriders in Infinity War, but some of Thanos' spooky alien soldiers survived the conflict. It seems like those Outriders are still loyal to the Mad Titan, because, in one of the recent Endgame trailers, Clint Barton can be seen running from some sort of monster with multiple limbs.

The shadow of this mystery creature appears inside of a burst of flames, and it looks identical to the shape of the Outriders from the last Avengers film. So even if Thanos doesn't bring back his Black Order, he definitely won't be alone in Endgame. 

RUMORED: ABOMINATION

Abomination fighting Hulk in The Incredible Hulk

Some fans pretend that The Incredible Hulk never even happened because it featured a different actor as Bruce Banner and didn't really contribute anything significant story-wise to the overall Marvel Cinematic Universe. But it's possible that the film's final villain may end up playing a major role in Endgame, especially if the Hulk refuses to help the Avengers fight Thanos the way he did in Infinity War. 

The Abomination is nearly as strong as the giant green monster, and he's far more ruthless. Emil Blonsky has been locked up ever since he disobeyed orders and tried to eliminate the Hulk, but if Thunderbolt Ross survived Thanos' snap, he could potentially release the Abomination and point him in the Mad Titan's direction.

RUMORED: ULTRON

One of the biggest complaints viewers seem to have about Avengers: Age of Ultron was that the "age" only turned out to be a matter of days. Ultron's reign lasts for a significant amount of time in the comic version of the storyline, but in the MCU, he was taken out before fans could truly feel like he was a significant threat.

Some viewers believe that we haven't seen the last of Ultron, though, and think that he'll return in Endgame. Thanos could bring him back to life with the Infinity Gauntlet, or one of the Avengers' trips to the past could prompt him to upload some part of his consciousness to a secret section of the Internet so he can reappear in the future.

CONFIRMED: "GREATER THREAT THAN THANOS"

Marvel's trailers for Avengers: Endgame have done a good job building up anticipation for the film without giving away too much of its plot, but their Endgame toys may have revealed a major spoiler. An action figure set featuring Thor and Rocket states, “The Asgardian Thor wields the mighty Stormbreaker in his battle against new foes. Rocket teams up with a new crew to guard the galaxy from an even greater threat," suggesting that Thanos won't actually be the ultimate villain of the film.

It's definitely hard to imagine that there can be a greater threat out there than an Infinity Gauntlet-wielding Thanos, but the Marvel Universe is filled with powerful antagonists. So count on Earth's Mightiest Heroes' fight continuing after the Mad Titan is vanquished.