The Venom symbiote is amazingly powerful, whether worn by Peter Parker or Eddie Brock, but it gets stronger in symbiosis with its host. Combined with a human, the symbiote becomes the Venom fans know and love, but combined with Marvel's Superman, it becomes a godlike army of Symbioids.
Venom has bonded with everyone from Deadpool to Captain Marvel, but the symbiote's most powerful host was inarguably the Void. The Void is the evil counterpart to the Sentry - a hero who has tapped into an ancient power equivalent to "one million exploding suns." Sadly, the Sentry's human form Bob Reynolds is unable to handle the sheer scale of his power, fabricating an equally powerful dark persona who answers his heroic deeds with evil actions. It was this demonic villain who unlocked the Venom symbiote's full potential.
Venom & the Void Are Marvel's Perfect Dark Combo
In Contest of Champions, by Al Ewing and Rhoald Marcellius, the Elders of the Universe have organized a huge competition in which each chooses a superhuman form Marvel's wide multiverse, with the final round coming down to two teams, captained by the Maestro (an older version of the Hulk) and the Punisher of 2099. After Jack Gallows' Punisher attempts to disintegrate versions of Venom and the Void, the two merge, creating a 'Symbioid' army. The Symbioids are an unstoppable swarm of Venom clones that are so powerful they can match blows with Blue Marvel and overwhelm the Maestro. Equipped with new powers, like flight, dark energy blasts, and nigh invulnerability, the Symbioids effectively mass-reproduce the Void's power, but this time without the Sentry around to oppose them.
Venom's Symbioids Are a Hulk-Killing Army
Each Symbioid was merged with a fraction of one of the oldest and most unstoppable evils of the Marvel Universe. The power behind the Void can be traced all the way back to the times of Moses, when it's implied it was sent as the final plague against Egypt. Similarly, the Venom symbiote is a product of the primordial darkness, shaped by the symbiote god Knull to help wipe out all life in the universe. Together, they effectively represent a threat entirely opposed to life itself, and they could easily have overrun the multiverse. Thankfully, the Elders' godlike ISO-Sphere managed to bring the Symbioids' hive mind under the competitors' control, and the energy manipulator Stick was then able to reawaken the Sentry, separating the Void and Venom back into their constituent parts.
It's always a surprise to see how a host's inherent abilities transform once they're infected by Venom, but in many ways the symbiote and the Void were meant to be, with both drawing power from the same cosmic darkness. Hopefully, Venom and the Void will never combine again, but in the vastness of Marvel's multiverse, anything is possible, and it's highly likely Marvel's heroes will one day have to reckon with a new Symbioid army.