AppearanceEditThe moment Dark Thralls are possessed by demons, they are severed twisted, mutating from their former selves to monstrous creatures with spikes, claws and horns growing from beneath their burnt skin.
From the concept art, one may see that a spike under the neck is called the "Teeth of Diablo". These may be the spikes described in the Dark Vessel lore, which perhaps provide access to demonic energies, or maybe control the demon once the summoning is complete.
Owing to their being possessed, they have limited to no control over their bodies. Their head rolls and swings in a very uncontrolled fashion and they have a general twitchy vibe about them. This is very apparent in their death animations, where they stand at the same place, with their entire bodies twitching and shaking, and finally explode into many chunks of flesh and bloody bones, letting the spirits that possessed them escape into the void.
Their mode of locomotion is quite strange, they can hover above the ground. There are not many monsters in the Diablo series that can hover without wings, only the Putrid Defiler does so. But the similarity in the lower torsos of both monsters is striking. They have their entrails and other innards hanging outside from their lower halves. How this aids them in hovering is not known.
Dark Thralls are mutated, half-demonic beings who fight furiously and move very quickly. It is a good idea to kill off the Dark Vessels before they can activate, if at all possible. There is no extra reward (and therefore no reason) to kill them after the transformation.
Once fully possessed, the Dark Thralls can attack by delivering melee blows for high Physical damage. Their movement is very fast, and if too far from their target, they may charge for even greater damage to everything in their path. This makes groups of many Thralls especially deadly: while player is locked in combat with some of them, others retreat and then charge in.
Upon death, they always explode, leaving no corpse behind.