According to Corey's prophecy shown on her tattoos when talking to Laurel Lightfoot while talking about Ian and Barley's quest, she explains that the Phoenix Gem in question is a cursed artifact and those who take the gem unleashes a Guardian Curse that will conjure a beast to slay those who dare take the gem. The only way to slay the beast is to stab its core with the Curse Crusher.
Role in the film
After inadvertently casting the Visitation Spell to bring their father back for one day, which is interrupted, resulting in only his lower half being brought back and their first Phoenix Gem being shattered, the brothers Ian and Barley Lightfoot go on a quest to find another Phoenix Gem to complete the spell and fully bring their father back before the spell ends at sunset. After an incident in the Manticore Tavern, Ian and Barley's only surviving clue they retained during the fire was a kid's menu indicating the answer to find the Phoenix Gem being at Raven's Point.
Ultimately, Ian and Barley survive a gauntlet of dangerous traps on their way to find the gem where they end up in the same high school Ian went to which Ian and Barley argue about the gem's existence. However, Barley encountered the same fountain he discovered when being prepared to be torn down by construction while Barley fends off the construction workers until he discovers a flower-shaped hole in the fountain and uses the rock he took from the raven statues at Raven's Point which unravels the Phoenix Gem. Upon obtaining it, the gem unleashes a curse and also causes an enormous dragon to appear. Barley later combines his late fathers' wizard staff using the gem to complete his father's full body before the spell fails at sunset while Ian, Laurel, and Corey defeat the dragon.
Ian and Barley use a Phoenix Gem in order to bring back their father; in mythology, a phoenix's life cycle has the phoenix be born again from its ashes after they age, which is why the gem is named after the aforementioned creature.