Nargul and Selune are a pair of male and female delvers you encountered when exiting the Vault of the Serpent Monks. They were hostile, and falsely believed they had discovered the passages you were in.  

Nargul appears to be a half-ogre fighting man of some kind, while Selune is a female tiefling adorned in leather armor.

When you met them, they were antagonistic but didn't press a fight, and indicated they would take their claim of trespass to the Delver's Guild.   Selune derisively labeled you the "Apple Tree Gang" when all of you but Carric refused to give them your names.

The pair accused you of "claim jumping", but a hearing at the Delver's Guild quickly revealed that they did not have the seniority to make such a claim.  In that incident, they locked you in a room with an enraged troll.

You next ran into them in Oldtown, during the attack of the mysterious white ape.  They tried to take credit for your work, and Yasin subtly suggested to the City Watch that they were up to no good.