Crater Assault Troops are one of the many infantry ranks of the Demonic Legion. They were first seen entering Sanctuary from the Arreat Crater, from which they derive their names from. They are seen in Diablo III.
The Crater Assault Troops are depicted as no other demon yet seen in the Diablo universe. However, there seems to be possible connection between them and the Horned Demons from Diablo I and the Siege Beasts from Diablo II: Lord of Destruction, as seen from their most striking feature: the single huge horn on their head.
They seem to have no eyes, or if so, their eyes have been covered by thick armor like the rest of their bodies. They are slightly taller and more muscular than the average man, as seen by the size comparison in its concept art. Being Assault Troops, they wear medium to heavy armor, that may cover their entire bodies or only certain parts. Their shoulderpads have a distinctive two curved spike style that matches with their headgear (or head itself).
Being strong fighters in melee combat, they usually fight with a weapon in each hand. Their concept art depicts them holding a pair of demonic mauls, but there is an axe and a military pick depicted nearby, which could indicate that the Assault Troops use a variety of Heavy melee weapons for combat.
The Assault Trooper seems to be capable of running at great speeds owing to its extremely muscular thighs and digitigrade legs with hoofs on its feet. Like most other bipedal monsters with digitigrade legs, it has to hunch forward in order to keep its balance.
Seeing the size and bulk of the Crater Assault Trooper, it can easily be compared to the Barbarian Warriors that dwell within the northern lands of Entsteig, where the Arreat Crater is located. There is a strange type of monster mentioned in the Barbarian's official lore entry, that describes monsters the size of Barbarians milling about on the slopes of the sacred mountain. It also queries whether the Barbarians have become so devastated by the destruction of the Worldstone, an artifact that they swore to protect, that they have lost their minds to the chaos.
It is entirely possible that the Crater Assault Troops have been mistaken for the aforementioned Barbarians seeing their comparable structure. But another seemingly far-fetched theory doing the rounds is that the Crater Assault Troops are mutated Barbarians that were affected by the Worldstone's destruction. Whether or not the above is true or if the Lore entry mentions an entirely new type of unrevealed monster is yet to be seen.