Initially a two boss fight, when one of those reaches 15% health, a third appears, so try to have both at the same health throughout.
They will call Valkyr to cast a spell occasionally:
Avoid black lines
Big purple hollow circles - no-one else stand in
Boss shield (blue runed "shield") - move boss out of shield area
Small purple swirlies with "flame" - 1 person soak each
Small white hollow circle - spread
Big light blue swirly with "flame", raid soaks
UNENDING STRIKE - tank swap when debuff expires, around 3 stacks
Formless Mass add - interrupt & kill asap
WINGS OF RAGE @ 100% energy - black hollow circle with suck - run out of circle.
REVERBERATING REFRAIN @ 100% energy - black hollow circle with push - stack in circle.
SONG OF DISSOLUTION - raid wide damage - interrupt asap.
SOULFUL BLAST - interrupt when possible.
When Skya joins BLOODLUST .. kill first 2 asap.
FRAGMENTS OF DESTINY - bouncing debuff - all affected stack together - dispel one at a time, person dispelled moves away. When final person debuff goes, patch on floor. Put patches to edges.
PIERCE SOUL - tank swap 2-3 stacks.
RESENTMENT - Raid wide damage. Heal through.