Shadow of the Vaal
Upon exiting the portico of the area, pulling the first pack of monsters will likely draw the boss as well.
This battle is the same as Vaal Oversoul with a few differences. The boss does not submerge for any length of time and may follow the target in the area. Chaos appears to be the main form of damage.
Three repeating abilities?
Launches an arc of shadow Magma Orbs in the targets direction.
Summons 5-6 Constructs.
Some projectile bolt ability? and spreads patches of chilling ground.