These use a ground targeted Fireball spell, each spell creates a wave of 3 projectiles. Additional projectiles add one wave, so each added projectile is 3 Fireballs added, creating VERY powerful overlap.
The Fireballs themselves have a little bit of damage, but the impact is where most of the damage comes from.
Sadly this spell is on a 2.5 seconds base cooldown.
They also have a physical damage Firestorm on a 6.5 second cooldown.
These would have actually been good picks, but their AI is probably one of, if not the worst I have seen so far.
These can be found in Sanctum in the second floor, they do not show up in you desecrate pool.
This video was recorded in patch 3.22.