Sulphur

Phantom Brave

30 years before the start of Phantom Brave, Sulphur appeared in the world of Ivoire and nearly destroyed the world if not for the valiant efforts of Scarlet the Brave.  Scarlet is able to banish Sulphur from the world and then mysteriously disappears.  
 
Just over 20 years after this, his presence begins to reappear throughout the world of Ivoire and his most powerful minion, Wraith, effortlessly kills the parents of main protagonist Marona and their friend Ash.   
 
Eventually, Sulphur returns to Ivoire on the Island of Evil.  Sprout engages in battle with Sulphur and seemingly defeats him and believes to have extracted his revenge, but instead Sulphur possesses Sprout and sends him after the party.  Sprout, realizing Sulphur is feeding off of his negative emotions, commits suicide to attempt to finally finish it off.  Despite this, Sulphur still manages to survive and attacks the party.  After being defeated and badly wounded, it attempts to heal until Walnut appears and appears to sacrifice himself to send Sulphur and himself away.  A stronger version of Sulphur, called EX-Sulphur, is an optional boss after completing the game.
 

Soul Nomad & the World Eaters

Sulphur is briefly mentioned in Soul Nomad when one of the supporting characters is revealed to be Walnut.  Walnut and Sulphur were found together drifting through a dimensional rift.  Walnut was pulled into the world of Soul Nomad and Sulphur is left in the rift.
 

Phantom Brave: We Meet Again

In the "Another Marona" story mode, Sulphur kills everyone in Ivoire.  Most of the supporting cast from the main story line appear on Marona's island as phantoms and tell her Sulphur has killed everyone except Marona, apparently.  Soon, the other Marona (who, to avoid confusion with that world's Marona, is named Carona) is given a few days by an Eringa that claims to be God to train the party to defeat Sulphur, and promises to revive everyone that Sulphur killed.
 
In truth, the Eringa is not God, but the "Merchant of Death", who has captured Sulphur and uses it as his ultimate weapon.  The party defeats Sulphur and the merchant of death, and everyone is revived.