Collect 1000 Seething Cinders from the Firelands.
Tarecgosa is bound to you, <name>, but only temporarily. An aspect's power is required to permanently seal her essence.
Malygos's successor will not be chosen until Azeroth's moons next eclipse, and the bond you share will not last until then.
Fortunately, there is a way to buy us the time we need. Powerful elements exist that can extend the ritual's effects. Some of these can be found in the Plane of Fire, with which you've become familiar.
Obtain them, <class>, and sustain Tarecgosa until the eclipse.