In this issue, one of the Slayers goes into a D&D-like underworld, which is filled with mutually hostile groups of monsters - fairies, slime monsters, some things that look like some kind of ogre/orc/troll type, and apparently a couple of others.
This underworld seems to literally be under the ground: she's shown climbing down, rather than having to go through a magic portal or anything like that.
The plot is that the ogre things are planning to invade the surface world, and if they do so will destroy the other species who live closer to the surface. Thus the Slayer has to convince everyone to make an alliance against the ogres. The whole setup seems very similar to various older D&D modules.