Silence of Lolth
You would think that if a totally evil utter bitch that EVERYONE else hates suddenly left everyone alone everyone would be happy right? Well, when that actually happened, not everyone was exactly happy about it.
The Silence of Lolth refers to a period of about 7 months (28 Eleasias 1372 DR to 28 Ches 1373 DR, by the Forgotten Realms calendar) during which Lolth cut off all access between herself and the rest of the Multiverse; her clerics lost their powers, she stopped answering communions, and plane-traveling to the Demonweb Pits in the Abyss became physically impossible. Naturally, without Big Momma Spider to keep their shit in check, this really fucked things up for the Drow, and their society started to crumble around their pointy ears; among other things, a patriarchal cult called the Jaezred Chaulssin - drow assassins with shadow dragon ancestry - started uprisings in four of the biggest Drow cities, accidentally caused another to be burned to ashes by magical "stonefire" (a sort of alchemical uber-napalm that melted even stone), and got Menzoberranzan invaded by Duergar and Tannaruks.
Things got so bad that the drow ultimately sent their own adventuring party to find a way to the Demonweb Pits to "rescue" her. It ultimately turned out that Lolth was building up her strength to seperate the Demonweb Pits from its own place in the Abyss so it could become an independent plane. Once she succeeded in that - and took the drow priestess who had found her for her new "avatar" - she stepped back in and promptly set everything back to business as normal.
The Silence of Lolth is the driving background event behind the "War of the Spider Queen" sextet of Forgotten Realms novels.