(I am the anonymous who posted this thread)

Your english should be fine.

Let's go with the 1 and 3 combined idea. Rather than being from a multiverse they can all be from the same fantasy world it takes place in. Perhaps they get killed by traps and monsters in addition to the main bad guy in charge of the dungeon (or he just escaped into the dungeon because he was going to loose to them otherwise). Although they mostly get split up individually and killed, perhaps sometimes they get killed in small groups as well and fall into a body pile. Either that or after they are killed individually they are dragged into a pile somewhere.

If they came in prepared they would have easily beaten the dungeon and the bad guy, but they were in a rush to go after him and was overconfident in their superior numbers and power.