To whom doesn't know Warhammer 40 000:
It's a wargame, a strategy game representing armies fighting, where troops are figured by models.
This one in particular has for background a very violent futuristic universe where humanity is controlled by a military totalitary system. It's engaged in a forever war against herself or one of many sadistic and aggessive alien races (what a kind place to be).
In this universe, the worst nightmares and thoughts of human spirit take form as Chaos creatures. These demons, in they turn, corrupt some humans to serve them. The army I'm presenting here is composed of these demonic followers.
Why choosing such a terrifying and unsane army? First of all because it's one of those which allow the biggest diversity of painting and modeling, so one of the most interesting technically. But also because it's a way to let steam off. It's a way, like hard-rock, to sate the violent part of me. And in term of humanity, I thinlk it's better than driving dangerously and scaring pedestrians...