It's only a black and white sketch, but it is hands down my favourite ever picture of a Pit Fiend. The way he's clinging to the mountain like that, and the baleful look in his eyes - he's the picture of malevolent, twisted cruelty.
DiTerlizzi has, in a previous entry, already said that he wishes he had had more maturity as an artist when drawing for Planescape, so he could have made an even better job of the Baatezu and Tanar'ri. He now looks to Bosch for inspiration, as the above picture shows. I especially like his comment that:
The most obvious flaw in the PS illus. is that they are all perfect specimens. After eons of fighting off adventurers, good guys from Mt. Celestia, and each other - well, they would show the effects of such a battered existence, and I think these wounds would add greatly to their grotesqueness.
He then illustrates what he means.
I'm sure I'm not the only one out there looking at those pictures and thinking, "I need to make demons and devils more like that in my games."