The nastiest demon lord in Nethack and Slash'EM, Demogorgon has the power to make you deathly sick when he hits you. If you do not have some means of curing yourself, you will die within a few rounds. Fortunately, Demogorgon is quite rare in vanilla Nethack; you may well ascend all classes without ever meeting him. In Slash'EM, on the other hand, he has his own level so you'd better be prepared for him.

To survive your encounter with Demogorgon, you will need some way to prevent or cure sickness. In vanilla Nethack, there is no practical way to give your character sickness immunity, but in Slash'Em, Necromancers and Undead Slayers are naturally immune to sickness. In addition, Demogorgon cannot make you sick while you are standing upon the mystic word Elbereth. The best way to cure sickness is to cast the spell of cure sickness at 0% failure. If you can't do this, you can use potions of extra healing, full healing, or holy water (non-chaotic characters only). Using a blessed unicorn horn is a bit risky, because if it fails you are likely to die from your illness. See "getting out of trouble" for more methods of curing sickness.

It's safest to kill Demogorgon without giving him a chance to retaliate. Because he is a demon, he is immune to death rays, but he can still be petrified or disintegrated like almost any other monster. Alternatively, wear a ring of free action, wield a potion of paralysis and hit him with it, then lay into him with your weapon. You may need to repeat this with another potion of paralysis to ensure that he remains paralysed until he dies. If you are a Drow, your attacks will put Demogorgon to sleep, but it is impossible to put him to sleep in any other way.

Engaging Demogorgon in honourable combat is not recommended, as far too many candidates for ascension have fallen victim to him. If you must do this, make sure you are very fast and unburdened and wearing an amulet of life saving. Use the upstairs/downstairs trick to stop him teleporting away to heal himself.