The monsters know what they are doing, sidan 70:
So here's our profile of drow combat: They patrol their caverns in darkness, using Stealth. They can detect enemies up to 120 feet away. When enemies come near, they fan out and Hide. When the player characters wander into their midst, one drow in the patrol (or two, if one is not enought) casts faerie fire to illuminate their interlopers, whereupen the rest attack with surprise, using their poisoned hand crossbows, and paying special attention to any dwarf in the party [w poison resistence]. Between the subterranean darkness and the drow poison, once the PCs get their chance to act in the subsequent round, many of them will have disadvantage on their attack rolls, while the drow will have advantage - an advantage they will retain even if the PCs fire up torches, lanterns, or light spells, thanks to faerie fire.
Det fortsätter så...