As Pele, trigger Everlasting Flame and survive 3 separate times in one match.
Everlasting Flame is Pele's passive ability. The description of this passive is "When Pele drops below 50% health her flame is set alight, and she gains increased Physical Power, Lifesteal and Ability Lifesteal. This passive has a 20s cooldown"
To unlock this, you have to reach 50% health for Everlasting Flame to activate. After it has activated, escape the fight you're in and survive. Best attempted in arena, where you can run back to the fountain and heal to ensure survival.
EDIT: Having just earned this, all you have to do is get to half health, either with damage from gods, minions, jungle camps or towers, to activate Everlasting Flame, Then run away and escape. After 20 seconds, repeat. Do this three times in a single game and it will unlock.