This is basically the premise of the BIONICLE serial "The Powers That Be". Kopaka and Pohatu are trying to figure out who's been murdering some of the most powerful beings in the universe, Including the local Lovecraft-Shout-Out, Tren Krom.
Though it's subverted in the case of Carapar, who tries to kill the Lovecraft Shout-OutTren Krom and gets disintegrated by the thing's Eye Beam for his efforts.
In another Bionicle serial, "The Kingdom", an alternate-universe Makuta Teridax has become Eldritch Abomination-like by absorbing as many of the other villains as he can, since in this universe, Mata Nui (and therefore, the entire Matoran Universe) died before Matoro could pull off his Heroic Sacrifice. Matoro eventually allows himself to be absorbed by Teridax, combining Heroic Sacrifice, Mind Rape and the Cthulhu-punch all at once; since Teridax couldn't take over the universe, he had nothing really left to live for, and it weakened his mind to the point that Matoro could make him kick his own ass.
The basic premise of the entire story, is its Big Bad, Makuta Teridax pulling one of these on Mata Nui, the Bionicle god, by poisoning him and casting him into a deep sleep, allowing him to rule over the Matoran people.
In the film Mask Of Light, Takanuva, the Toa of Light fights Teridax (who is basically Satan) and wins by pulling off his mask and dragging him into a pool of protodermis, fusing the two of them together. They emerge fused together as a single being named Takutanuva with Takanuva in control due to Teridax being weakened by the loss of his mask. They get seperated when they're flattened by a giant door. Long story. Anyway, Takanuva is resurrected but Teridax survives as a spirit.
Later he gains the powers of a God and becomes ruler of the Matoran Universe, later having a showdown with the Matoran god whose power he stole, Mata Nui. Despite now being the less powerful of the two, Mata Nui defeats Teridax by pushing him into the path of a speeding moon.
The Toa Metru and Toa Hagah both managed to defeat the Makuta as well. Unlike his later defeats by the Toa Mata and Takanuva, Makuta's Gambit Roulette did not have his defeat planned for at the time. Especially noteworthy is that the Toa Metru were highly inexperienced at the time, though they did get a massive power up in the Mask of Time.