Drow and The Underdark

===Note on the Drow on Dalelands===
There are a lot of them. This is lore. 
According to the FR Campaign Setting 3.5, 47% of Cormanthor is made up of Drow and from Fr: Shadowdale, the Scouring of the Land there has been a long history of the Drow in Shadowdale itself. Indeed, at times it has been under their control, though not for a while now.
===How should you react to meeting a Drow? ===
You should be scared witless or be ready to kill!
Drow are a Kill On Sight monster race on the surface. You should no more make pals with them then you would a gnoll.
Can you have a good drow friend? Yes. But it shouldn't be easy for them to gain your trust. Mistrust should be the first reaction. (quoted from here).
==The Shadowdark, the Drow and Shadowdale. ==
The Shadowdark is approximately 120 miles in diameter and centred under Shadowdale village.
Once long ago the Drow dominated Shadowdale, building the Twisted Tower and keeping the surface folk as their slaves.
For centuries since that time the drow of Maerimydra have employed the tunnels beneath the Twisted Tower as a base from which to launch attacks on the elves of Cormanthor, only to be driven back into the depths time and again. In -331 they were heavily defeated and the Twisted Tower was left in the hands of good-aligned dark-elf allies, who transformed it into a great temple to Eilistraee. Later it came back into Drow control and then finally into Human hands when the Human slaves rose up in rebellion.
Temple of Lolth
House Dhuurniv has fallen in and out of favour with the Spider Queen. Currently in favour, the clerics offer frequent sacrifices on the altar in hopes of keeping the Spider Queen's favour.
==Maerimydra==
Lost in religious war during a period when Lolth was silent to her followers, Maerimydra was once the greatest drow city of the region. A powerful priestess of the drow goddess of the undead (who was slain by Lolth long ago) now controls the city, aided by goblins and fire giants. 
In a little more detail, when the Silence of Lolth began it caused growing unrest among the Drow until the city's archmage, Duneth Wharreil, used the confusion to launch a coup on the remaining houses. The same year the city was sacked by the half-fiend Kurgoth Hellspawn and his army of goblins, ogres, fire giants and demons. Most of the drow inhabitants of the city were killed or enslaved, with some escaping into the surrounding Deep Wastes. The single drow house to survive with any assets was House Dhuurniv, because most of them lay outside Maerimydra. House Dhuurniv migrated to the Shadowdark under Shadowdale village while rebuilding its strength.
Fire rules supreme in Maerimydra now. Giants have cemented their control under their queen, the fire titan Hledh. The giants and their servants have rebuilt most of the city to proportionate size and expanded the vault in which it was built. Azers and hell hounds patrol the nearby Underdark, and the giants keep hundreds of drow as slaves.
Approximately 2,000 refugees made their way to the Shadowdark from the ruined city. Many hope to form a new settlement under the leadership of House Dhuurniv. Now the underground drow have re-established themselves in the Shadowdark beneath the Old Skill hill in Shadowdale.
==Surface Drow==
There has been a very strong presence of Vhaerunite Drow for a long time in Cormanthor and the Dales and they are fiercely opposed to the refounding of Myth Drannor. Notable Vhaeraunite names include the Auzkovyn clan and House Jaelre.
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Sources
Forgotten Realms - Shadowdale - The Scouring of the Land
==Eilistraeens==
There is a temple not far from Myth Drannor in Velarswood: Harrowdale's woodcutters and loggers stay on the fringes of this ancient wood. Cloakers, trolls, stirges, bonebats and more dangerous predators hunt the interior of the forest, and are hunted in turn by worshipers of Eilistraee who have a hidden temple in the wood's northern reaches. The drow who worship Dark Maiden spend much of their time in Cormanthor now, fighting against the Vhaeraunian drow, but they don't leave the temple unguarded. ( source: Forgotten Realms Campaign Setting).
Note: Without the Eilistraeens, the Elves of Myth Drannor would be under more attack from the Vhaerunite Drow.
==Note on Shevarashans and Eilistraeens:==
1.) Shevarash hates all Drow. His followers are not wont to form friendships with Drow whatever their alignment or religious bent.2.) Any truce mentioned existing with Eilistraeens is simply a non-aggression agreement, not an alliance or implication of harmonious cooperation.3.) Mistakes happen. Shevarash and his followers do not approach Drow expecting them to be good, or giving their hated enemy a chance to strike first.
"The Black Archer has no patience for those who do not share his zeal for vengeance, and he has no interest in moderating his crusade in the interests of peace".
[quoted from forums here, last post, credit Crispy Walrus, slightly altered]
In summary, here on Dalelands the Shevarashans follow one premise: A good drow is a dead drow!
A follower of this elf god would much prefer to kill drow first ask later. While he would not go out of his way if he gets wind of a gathering of Ellistraeens to slay them all he also wouldn't belive for a moment ANY drow is up to anything but no good.
Forum Quote
Sources
Forgotten Realms Wiki: Maerimydra
Forgotten Realms - Shadowdale - The Scouring of the Land
Forums.