Zurvan

Zurvan (or the Ancient One) was a primal zerg of Zerus. Zurvan was spawned before the Overmind, and lived to see the return of the zerg Swarm to Zerus during the Second Great War. It shepherded a deinfested Sarah Kerrigan's rebirth into the Swarm, before being killed and having its essence absorbed by her.

Biography
Zurvan was younger than the power found in first spawning pool but older than the first Overmind. It escaped being incorporated into the Swarm hivemind and remained on Zerus when the Swarm left the planet. In time, it became a pack leader and became immense both in power and physique. By the time the Swarm and a deinfested Sarah Kerrigan returned to Zerus, it had been millennia since Zurvan's last evolution.

On the advice of Zeratul, a protoss Dark Templar, Kerrigan sought out Zurvan to help her claim the power of primal zerg. The Swarm fed Zurvan quillgor to awaken it. The primal Brakk and its pack unsuccessfully attempted to halt the awakening. Zurvan instructed Kerrigan in the ways of the primal zerg and the ancient history of the zerg. Most importantly, it directed her to use the first spawning pool to transform into a primal-based human-zerg hybrid ; the new form was more powerful than the old Queen of Blades.

When Kerrigan came to Zerus, she was far less powerful than Zurvan. Once a reinfested Kerrigan killed and absorbed the essences of the other pack leaders, she became worth consuming. She expected Zurvan's challenge, and killed and absorbed its essence as well.

Gameplay
Zurvan appears directly after killing Slivan and receiving units to control. He will burst out of the ground, leaving a crescent shaped battlefield with his head in the center and tentacles on the exterior. The first stage consists of him smacking the ground with multiple tentacles dealing large damage, and spitting out Zerg minions.

The second stage which comes up when he's at about a quarter health consists of him blasting a third of the stage, leaving energy pools that damage you. His attacks alternate between each third of the stage, and they take some time to charge, giving you time to avoid it.