The Wish Seed is a seed said to have the ability to grant any wish. This seed is found on Phoenix Mountain and is protected by the The Immortal 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 civil war. In reality the Wish Seed is a fake and is eventually destroyed by 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. The countless deaths caused by this was all a part of Hades's plan to create more monsters for his army by using the souls lost. In Chapter 11, Viridi unknowingly places the blame on the humans rather than on Hades, so as punishment, she throws a nasty Reset Bomb to make a stop to the fighting.
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.