The 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 change.
During the time of Urgence Evergreen, the comet appeared green, and it was known that on this occasion its impact would kill even the 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 wizards. Evergreen responded by freezing the other Elementals and proceeding with his plan, but apparently met with failure due to his apprentice Gunther donning the crown and losing his sanity. This comet has horn-like flames, which could be associated with The Lich.
Sometime before the rise of human civilization, 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.
A blue incarnation of the comet was a past life of Finn. It is not known whether he was one of the previously seen comets or a separate incarnation.
A thousand years before Adventure Time's era, during the Mushroom War, a nuclear bomb detonated and brought destruction and the Lich's presence to earth. It is unknown if another comet had something to do with this event.
Almost a thousand years after 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.
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 slayed 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 possibly become a god. Finn denied this request, and Martin ended up accepting this favor instead. Both of them disappear. It is still unknown if this comet has brought any change to Earth - if at all, what with it never colliding with the planet.