Upon defeating the Ancient King on Floor 10 of the Labyrinth you are given the choice to enter "The Palace of Karma" the PVP grounds of Buriedbornes.
Here you will not actively fight another character, but a copy of one that was recently registered into Karma (i.e. asynchronous PvP). Each room contains an enemy player, and some rooms have special effects like a regular dungeon.
Should you hit floor 20 (20 wins in a row) you can earn 2000 Fragments of Death for a single run!