Sepulcher of the First Ones is one of the three raids that are available with fated affix during Season 4. All bosses will drop relevant gear with upgraded ilvl. Ranging from 278 in NM, 291 in heroic and up to 304 in mythic, buying full gear in Sepulcher of the First Ones is the best way to boost the character’s power in S4.
This offer includes items not only from Sepulcher of the First ones but from two other raids (Sanctum of Domination and Castle Nathria) as well. Only one fated raid is active at a time, so it is irrelevant to try to farm gear only from Sepulcher of the First Ones as this raid is available onlyonce per 3 weeks.