This prompted me to start Dungeon of the Endless, and even after going through the tutorial I'm still not 100% how the game works. Like, when you've explored all the rooms and are ready to move to the next floor, you go back to the first room and interact with your crystal. This starts the final waves of enemies. Do I try to move through the dungeon with the crystal and try to survive or do I just weather the waves of enemies and they eventually stop?
You have to pick up the crystal and move it to the exit, and all players have to be in the exit room to progress to the next level. I usually plan out my exit path and, kind of like in a tower defense game, place items along those paths that will slow down or take out enemies as you are moving. Each character has different movement speeds, so sometimes it helps to have the fastest character pick up the crystal.
Once I understood the mechanics, I thought the game was very addicting. Have fun!