The boys continued down in Undermountain, to the north. I set a drow to shadowing them, making mischief.
They saw torches in the distance (Dancing Lights, to draw them into a dangerous room). Younger son, the elf mage, surprised me by sneaking around the long way, and actually saw the drow from behind (barely visible as an unidentifiable humanoid figure in the near total darkness). He retreated to tell the dwarf fighter what he saw. By the time they returned, the figure was gone.
They were creeped out, and reversed their direction, and explored elsewhere. They stumbled into a room with 7 animated skeletons, and fought them. In the middle of the fight, everybody – skeletons included – started glowing with a dim blue light (Faerie Fire), which made the fighting more deadly for all.
Dwarf and elf were wounded, and ill-at-ease with the strange occurrences, so they elected to retreat back up to the Yawning Portal to rest. I was going to keep having the mysterious happenings continue, but I forgot my younger son – the elf – always likes to chat it up with the inkeep and adventurer patrons at the inn. He recounted the events quite thoroughly, and the patrons identified their stalker as a drow elf.