A Catalyst Comet is a massive comet that hits the Earth every 1000 years. It is revealed in "Evergreen" and "Astral Plane" that the comet has hit the planet repeatedly, always bringing with it some kind of agent of change.
During the Cretaceous period, a green incarnation of the comet approached Earth, and it was known that on this occasion its impact would cause massive devastation, even killing Urgence Evergreen and the other Elementals. Evergreen sought to prevent this catastrophe using a magical crown that he had created to grant its wearer's deepest wish, but was opposed by his fellow Elementals, as its power would cause irreparable damage to existence. Evergreen responded by freezing them and proceeding with his plan, but met with failure due to his apprentice Gunther donning the crown and losing his sanity as he turned into what he saw Evergreen as: an ice wizard who ordered people around. The comet struck, and presumably caused the extinction of the dinosaurs. This comet contained the essence of the Lich, as the incarnation of pure evil, as it had horn-shaped flames, a "desire for extinction," and it was green, just like the Lich's magic.
Sometime thousands of years before the main Adventure Time era, a yellow incarnation of the catalyst comet appeared and Orgalorg, the Breaker of Worlds, attempted to absorb it and gain its power. Abraham Lincoln disapproved and asked Grob Gob Glob Grod to intervene. Grob Gob Glob Grod incapacitated Orgalorg before he could intercept the comet and the latter fell to Earth, where he would later become Gunter.
One of Finn's past lives was a blue incarnation of the comet, which struck Earth at a currently unspecified time. The comet was likely Finn's first life, as it was the first life mentioned in "The Vault." It’s likely that this was done by whatever cosmic entity sends the comets as an attempt to balance out the Lich’s existence. A being of pure evil was in Ooo, so a being of pure good must exist as well.
Approximately a thousand years after the Mushroom War, what appeared to be another blue Catalyst Comet came hurtling towards Earth, but was noted by Grob Gob Glob Grod as being off in its timing as well as on a trajectory towards Mars rather than Earth. It was diverted back to its proper course by the sacrifice of Grob Gob Glob Grod and was revealed to in fact be a spaceship piloted by Martin. Despite not being the actual comet, this event caused repercussions on both Finn (who visited his astral plane) and Sweet P (who dreamed about it, as he's the current form of The Lich).
Not much time later, the real comet—this time purple in appearance—came towards the earth. Orgalorg, resurging from Gunter, tried once more to absorb it, but Finn slew him right after he had swallowed it. The comet is revealed to be an old god-like entity, and reveals that Finn is one too. The comet finally gave Finn the option to either stay on Earth, or go off and live in another form. Finn denied this request, and Martin ended up accepting this favor instead. With Martin discorporating, both Martin and the comet disappear.
- Finn states in "Crossover" that he has a birthmark shaped like a flaming sideways teardrop. This is a reference to his past life as a comet.
- In "The Comet," the purple comet says that it has embodied all that is good and evil. While it says "good," the shot focuses on Finn and when it says "evil," The Lich appears on screen.
- This, along with the Evergreen comet's appearance, and Sweet P's dream in "Be Sweet," confirm that the Lich was that incarnation of the comet.
- During the Mushroom War, a nuclear bomb detonated, bringing destruction and releasing the Lich's essence to the land. Given that the war happened about 1000 years before the present Ooo era, the series' timeframe suggests that a comet should've hit Earth around that time, though no direct mention of it has been made.
- However, in "Finn the Human," when Farmworld Marceline explains that reality's version of events, a blue-ish comet is seen passing next to (but not hitting) the frozen Farmworld Earth after the Mushroom Bomb was stopped by Simon Petrikov.
- Also, in "I Remember You" and "Betty", a picture of a comet approaching Earth can be seen on the bottom-left corner of the Ice King's "past room."