There aren’t that many options that focus on Warlocks specifically. If you’re fine with worshipping the Legion and being an antagonist to the Horde and Alliance there’s the Shadowbane Coven:
Your options become a touch more plentiful when you decide on what race you will play and how you plan on conducting yourself.
There are guilds like House Bemoux and The Cursekeeper Association ([N-RP] The Cursekeeper Association - Now recruiting!) that would suit warlocks who are less inclined to plot world domination and more interested in the scholarly, occultist aspect of the class, but they are not covens or limited to warlocks.
You basically get a plethora of options for guilds that accept warlocks (and utilize them) but have a very limited selection if you want something that strictly focuses on that, to the point there’s sadly no Alliance or Horde-Aligned “Warlock Guild”.