Hrm. It seems that the Underdark was not quite so good an opening point for invasion to Toril after all. The setup was lovely, mind, with such an arrogant drow queen who would happily dance on an Archdevil's string unknowingly making everything fall just so. It worked without a hitch, that adventurer that found his artifact on the Plane of Shadows has neatly taken his place in Cania, it's just the whole... 'invasion of Toril' part that's having some trouble. See, his (invincible, infinitely replenishing, really quite cleverly crafted) soldiers are essentially immortal (in that while they can be killed, it doesn't keep for particularly long), they are still vulnerable to such things as bottlenecks. There is really only one good exit from the Underdark for an army, and rather unfortunately for him, some epic level adventurers are parked on it and determined to make him work for every last inch.
Annoying flies. He'd kill them himself, but really, why go through all the trouble? He's out of Cania, unbound by the machinations of the good high powers that keep him and his like at least somewhat contained, so if this place is giving him trouble, why not just go and do a little bit of window shopping. He's not particularly attached to conquering Toril to be his new domain.
And on another plane, far away, some foolish old king was shortsighted enough to make a door. How polite! How short sighted! He thinks he'll use it, thanks. He was getting sick of all of those drow, anyway. Best to have a change of pace, stay on his toes. Well, cloven hooves. Whatever, it's a turn of phrase.
Hello, Osirion! Would you like an army of invincible undying soldiers in your lovely lawful little nation? No? Too bad, you were the ones dumb enough to put a door to an evil afterlife here, you can't expect that someone won't decide to come visit. Mephistopheles announces himself in the way of grand Archdevil tradition. Rain of fire, swarm of locusts, transparent tortured soldiers marching across the lands and slaughtering anything that dares stand against them, dragging anyone that doesn't off to become soul fuel for his lovely, lovely war machine. The usual. Wouldn't do to lose sight of the classics, after all. They're why devils are some of the most fearsome evil beings around. Neatly organized, terrifyingly efficient.