Every one thousand years, Mortos rises from the "Spooky Realm" to grant one wish for one mortal before he returns. Dib, being a paranormal investigator, went to the graveyard where Mortos awakens in order to ask him for a wish; that wish being that Mortos steal the soul of Dib's archenemy Zim.
But when Mortos arrived and befriended Dib, he needed to rejuvenate his supernatural powers by eating junk food, buying clothes, and hugging puppies. During this, Dib became annoyed at Mortos's child-like behaviour, ridiculous means of strengthening, and his general idiocy. When Dib declared Mortos to be a fake, the creature became angry, and used his rejuvenated powers to create fires, destruction, and chaos, even transforming carnival rides into living monsters. After Dib saw this, he was thrilled, and believed in Mortos's power. But it was once again drained, and Mortos needed to strengthen himself again.
When they ran into Zim, who was in the middle of an "evil" scheme, Dib began to fight him. This was just when Mortos's power built up again, but Dib was too busy fighting to notice. So when Mortos heard the wish of a random guy passing buy, (that wish being for ice cream), he granted the wish and returned to his realm, much to Dib's dismay.
While not completely evil or antagonistic, Mortos possessed dark powers, which he could and would use for evil purposes.
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