The Amalgam of Rage is a demonic fiend created by Lilith from Nafain's blood. It represents Nafain's hatred of the Pale Knights, and the Cathedral of Light for their part in making the culture of druidism to be forgotten in Scosglen. It was gifted to Astaroth to use as a mount.
Biography[]
"No. She was already gone. And I fear she has taken a piece of you with her... a beast born from your own blood."
- Discussion between Nafain and the Wanderer(src)
In order to find the burial site of Astaroth's soulstone, Lilith payed the druid Nafain a visit. Playing on his hatred of the Pale Knights, she made a pact with him, and agreed to wipe out the Knight Penitent at Cerrigar; Nafain's hometown. In a means to kill two birds with one stone she cut the druid in two, and bound his torso to a tree in the moors at Daudur Peats. The tree corrupted his body and prevented him from dying from blood loss. His blood flowed through area, corrupting and/or killing the flora and fauna within its reach. Lilith channeled the blood and corruption to a nearby cave, where they began to gestate into the amalgam.

The Amalgam, just after exiting its gestation cocoon
The Wanderer disrupted the gestation before it was completed. Fortunately, it was almost completed and the Amalgam of Rage emerged, with bone and sinew partly exposed. After it was injured by the Wanderer in a short battle, Lilith recalled it.[1]

The Amalgam (in the background) and Lilith, as she bargains with Astaroth
Lilith led the beast to attack Eldhaime Keep. After tearing right through its defenders, Lilith led it to Astaroth's soulstone beneath the keep.[2] When Astaroth was freed, the Amalgam was gifted to him as a mount. She then took them to Cerrigar. This was to make Astaroth fight harder to consume his host, and to complete her pact with Nafain. The amalgam, with Astaroth as its rider, decimated most of the knights at Cerrigar (and the rest of the town). It was later killed along with its rider by the Wanderer.[3]
In-game[]
The Amalgam of Rage is a boss monster in Diablo IV. It mainly attacks with bites, claw swipes and pounces. It also has a terrifying howl that summons nearby canines to fight for it. When being ridden by Astaroth, the demon can make his mount breath fire from the mouths of its three most prominent heads by channeling his flames through it.