The Cromulon invasion from Rick and Morty's 'Get Schwifty' saw a race of seemingly unkillable aliens force the planet to participate in a cosmic singing contest under threat of destruction. However, while the vast Cromulons were able to withstand nuclear weaponry in the episode, it turns out there is someone who could have destroyed them... if Morty hadn't killed him a little while before they arrived.
The Cromulons are an alien race who run Planet Music, a genocidal singing reality show where being 'voted off' results in having your planet vaporized. In the TV show, the surprise deaths of America's greatest performers leaves Rick, Morty, and Ice-T to come up with a song that will save the planet. However, had Rick been able to call longtime contact Krombopulos Michael, Earth could have been saved by the overzealous assassin. Unfortunately, Morty had already killed Michael in an attempt to save the extradimensional being Fart, who he ended up killing anyway.
'The Life and Death of Krombopulos Michael' from Rick and Morty #34, written and drawn by Kyle Starks, explores the killer's life before he was crushed by Rick and Morty's spaceship in his fateful last mission. The story offers explanations for Krombopulos Michael’s name, his business relationship with Rick, and the identity of the woman pictured in the locket he looks at after praying. However, it also shows him out on missions, one of which is to destroy a Cromulon who is in the middle of bellowing "Show me what you got!" to the latest Planet Music contestant planet. This shows Krombopulos Michael is ready, willing, and able to destroy the otherwise invulnerable race, and could potentially have saved Earth if he hadn't perished a few episodes before their arrival.
Krombopulos Michael is able to implode the Cromulon using a custom weapon it's implied he got from Rick. It's unclear why Rick wouldn't manufacture this weapon again once the planet came under threat, though many possibilities exist, from lacking the time or resources needed to create it, to believing in his songwriting abilities enough to want to stay in the competition, to simply considering the confrontation too much trouble and planning to ditch Earth to its fate as he has before. However, had Rick been able to simply contact Krombopulos Michael with his pre-existing weapon, it's likely he'd have taken this low-effort out, showing how the Cromulon threat (and the cult it inspired on Earth) could have easily been dealt with if Morty hadn't accidentally instigated a killing spree in season 2, episode 2 of Rick and Morty.
The main joke of Morty killing so many people to save Fart is the unpredictable nature of the universe, as he does incredible evil in pursuit of the greater good, only to discover Fart is a threat to all carbon-based life and has to execute him just as Michael was originally hired to do. The fact that killing the amoral assassin Krombopulos Michael also left Earth vulnerable to later attack by the Cromulons only intensifies this dark joke, and shows that in the universe of Rick and Morty, competence trumps good intentions every time.