Completing Phosphor's questline will require you to complete the (4) quests that have the phoenix icon located on the map. The first will be to acquire Phosphor as a Godly Power, and the last will have you face off against the Griffin boss. (Trying not to spoil too much).
Here are the names of the required quests in order:
This is definitely one of the shorter and easier questlines in the game. Plus you'll receive some cool looking cosmetics upon completion.