The following information on the Lord of the Second, His Grace, Archduke Dispater, is based upon information compiled over the three editions of D&D.
Dispater is one of the oldest of the Arch-Fiends and has always ruled the second circle of Hell, Dis, as far back as anyone can remember (including gods). Dispater is perhaps the most "approachable" of the Lords of the Nine as the relatice accessibility of the city of Dis attests. Dis is the largest known city in Hell and has sections that cater to the lifestyles of non-Planar beings, including mortals like Humans. Dispater rules the city and the layer from the Iron Tower, which can be seen from any point in the city regardless of the direction the viewer faces. While in his Tower (and some would argue while he's on the Second), Dispater is almost completely immune to any and all forms of attacks. Needless to say, he rarely leaves the tower.
In the politics of Hell, Dispater used to align himself with the Lord of the Third, His Excellency, Count Mammon, and with the Lord of the Eighth, His Grace, Archduke Mephistopheles. Some claim that this alliance is due to the fact that both these arch-devils, like Dispater, are "true" devils, while their rivals, the Lords of the Fourth, Sixth, and Seventh, are fallen celestials. Others claim that such is not the case and that all of these beings are devils and that Mephistopheles has promised the other two something Baalzebub (Mephisto's rival) can not. The two factions hope to destroy the other, the winner thereby acquiring enough power to topple The Lord of the Nine and the Overlord of Hell, Asmodeus. Eventually, this conflict reached a point in which it appeared that eight of the Nine Lords (all but Asmodeus) would fight openly among each other when, in a brilliant turn of events, their own armies, still loyal to Asmodeus, turned on them and cast them down. Asmodeus quickly reinstated most of the Lords to their former positions. Since these events, Dispater does not trust Mammon (who groveled before Asmodeus) and he has no intention of openly supporting Mephistopheles unless the Lord of the Eighth has a fool-proof plan.
Dispater is the chief antagonist of a great Planescape module, Fires of Dis. His appearance in the module is great and I would strongly encourage that you track it down.
