The steed has has some extensive greenstuff work applied to it (or milliput in this case). Lots of detail has been added to the surface to demonstrate a diseased horse with decaying and rusted armour. This technique has also been extended to the legs of the chaos lord to give a more Nurglesque appearance. The horse also has eve guards that are similar to the destroyer hive accessories that Typhus carries around -- very fitting for this fellow.
The torso of the chaos lord is one of the Forge World Death Guard / Plaguemarine conversion set. He is armed with a power fist (chaos space marine) and a wicked looking halberd (chaos knights). The toughest part of the conversion was getting the torso to sit on the chaos knight legs right. Lots of re-positioning was needed to make the angle of the halberd and power fist look fluid.
In game terms, this is a chaos lord with the Mark of Nurgle, equipped with a power fist and a close combat weapon (halberd!). His role is to take care of some of the hardier targets, terminators and so forth, before the bulk of the army has to worry about them. I considered adding some melta bombs as well for extra anti-tank duty, but felt that the powerfist was okay for a bit of light tank busting. He's therefore a threat to most opponents in the game: not an over-powerful opponent, but one that merits attention.