The Swarm is a Fallen in Oninaki. It is the boss of Kiraq and is fought during Chapter 2 Part 3. It can also be fought on level 50 of the Sanctum of Rebirth.
Log entry
Horrific Fallen comprised of myriad masked countenances that unleash a unified onslaught once each face is ready to strike. The beams they shoot out from their spherical formation are hard to evade without jumping, but a broken face cannot fire, so focus on just one.