Erodon is the true final boss of Crossover X, and the crossover universe's former god of destruction, prior to an unnamed new one's arrival. See, when a new god or goddess comes to take an old one's place, the old one begins to slowly vanish, becoming one with the world around it. When Erodon discovered this fate was bestowed upon him, he put on an amulet to slow his fading.
After being defeated, he goes into a very Kefka-esque speech about no longer understanding the purpose of life (meaning he's not malevolent at all, only depressed) and says that if he were to die, he'd make the rest of the world go with him. He then breaks the amulet around his neck, drastically speeding up his fading and putting himself out of his misery.
As you'd expect from the true final boss, Erodon puts up a very difficult fight. He's not very big (justified, since he supposedly shrank himself) and fights like a watered-down Jasper Batt Jr. form 2. He does spam teleporting to an extent, but doesn't punch you whenever you come near. He also has a crapton of elemental ranged attacks. He uses the fire-based one the most, so don't fight him with Ruby! On his last slice of health, he begins using a move that combines all of the elements he uses into a giant beam.
The music that plays in this battle is the final boss music from New Super Mario Bros. Wii. Listen:
The attempted mood here was a song that is both epic and sad, to represent Erodon's current sate of mind.