A nihilist ending is a type of "victory" that involves destroying the universe. In terms of points it is about as rewarding as a neutral ultra ending.
A nihilist ending requires entering the chaos gate. This can be accomplished either with the Ordinary chaos god ending approach, or the Ultra ending approach. Once on the plane the PC must read a scroll of entropy while standing next to Andor Drakon. This instantly kills him and initiates the ending.
End of the world
Once Andor has been destroyed, a portal leading back to D:50 appears. Parts of levels will begin being replaced with black patches of nothingness. Walking into one of these will result in a death instead of a victory. Revisiting a level the PC has been on since initiating the ending will result in a death instead of a victory.
Each level is only safe to stay on for around 17 turns, although failing to leave after this deadline will still result in a victory. Teleportation is allowed on all levels, and in practice is necessary to beat the clock. The D:32 shortcut is not accessible, so ascending all the way to D:1 is necessary to observe the ultimate destruction of the Drakalor Chain. The gods can die at any point during the ascent, which prevents any prayers from working.
Note that it is necessary to close the Chaos Gate in the 17 turns before ascending to D:49, otherwise the stairs on D:48 will remain facing downwards and the player will be trapped.