The Chosen Ones

To check it: This potion increases your mana permanently by 50 points.
A normal mana potion (which can't be used in this quest) restores mana by X points. You notice the difference?
The Nightmare Puzzle Doomlord's talking a bit is in the southeast corner of the map, near the teleporter. You walk to that place and suddenly a message appears "with which character do you want to join the Nightmare puzzle?".

The puzzle you've solved isn't the nightmare puzzle, but some kind of labyrinth.