How did Drow get into UO?
Drow got to Sosaria (the planet of UO) the same way Lord British and the Avatar got there – through a black gate/portal. Lord British departed from Britannia through one of those not long ago. Assuming black portals lead to the shadow plane, which is filled with portals to other planes and realms, they provide rather easy, although dangerous, way to travel bethween Faerûn and Sosaria. The lower Underdark is of Faerûn is full of those with appropriete nasty stuff waiting behind them for a dinner of tender adventurer meat. There are also a couple more convinient placed ones – of course such gates are usually well-hidden and heavily guarded.
