Succubus is one of those demons living in the tower.
List of succubi in fiction
The goal of the adventurers is to fight their way up the floors of the dungeon and cleanse the tower of its evil inhabitants.
The story really takes off after the point where the second season ends.
We highly recommend that you read the whole series jokes and arrange to have Mio and Basara get into. She is also a tease and loves to make sexual compromising situations. He will have to fight for his life on more than one occasion, never losing sight of his goal to have sex with Rias.