The Wish Seed is a seed said to have the ability to grant any wish. This seed is found in a volcano and is protected by the Phoenix. Hades tricks the desire-driven humans into thinking someone took the seed and as a result of this, the humans begin slaughtering each other in an all out war. In reality, the Wish Seed cannot actually grant wishes (as something like that can make a being more powerful than the gods) and is eventually destroyed by either Pit in Chapter 10 or destroyed by the Underworld Army (by a Skuttler trying to whack it into a Magmoo's mouth) depending on whether Pit destroys it in the battle against the Phoenix or not. Because of the war over the Wish Seed, Viridi is angered at the humans for their foolishness and sets off a Reset Bomb.
If the Wish Seed had one wish, it would probably be to actually have the power to grant wishes. But unaware it's a fake, humans and gods alike blindly seek to possess the Wish Seed, even facing off against its defender, the Phoenix.