The Beast

The Beast was an Ork warboss that threatened the early Imperium of Man with the single largest war since the Horus Heresy.

Official Canon
The Beast first appeared in M32 when the Imperium had started To recover from the Horus Heresy. At the time, it’s military power had started to die down, leaving it vulnerable when the Beast and his massive armies attacked. The Beast himself is gigantic, powerful, and extremely cunning, of the same type as the warboss of Ullanor that Horus defeated. The Beast was eventually confronted by Vulkan, during their final battle on Ullanor. Though Vulkan Killed the Beast by pushing him into the Temple Gargant’s reactor; the explosion in turn killed Vulkan.

TTS Canon
The Beast appears as one of Horus’ allies in the April Fools special.

The Beast is only mentioned indirectly by The Fabulous Custodes when they nervously tried to avoid telling the Emperor about the Beast’s invasion. Vulkan also mentions his battle with the Beast, and how the explosion caused him to become more Orky, perhaps absorbing some of the Beast’s personality.