Add a system of “hell curses”. When you enter a demon lord’s lair, you are afflicted with 2 random curses, which stick around forever until the lord is killed or bribed off.

You can receive at most six curses over the course of the game, requiring you to defeat the lords of the first three lairs you encounter. Further generation of lairs does not add any curses.

A non-comprehensive list of potential curses is:

• healing potions heal only half as much HP
• your speed is knocked down by 2
• your AC is deducted
• your spellcasting failure rates increase
• you are randomly inflicted with confusion every 50 turns or so