If your character has killed Sauron (a difficult task), who lives on level 99 (4950’) in the dungeon, a magical staircase will appear that will allow you to finally reach level 100. Morgoth lurks on this level of his dungeon, and you will not be able to go below his level until you have killed him. Try to avoid wandering around on level 100 unless you are ready for him, since he has a habit of coming at you across the dungeon, the Mighty Hammer ‘Grond’ in hand, to slay you for your impudence.
If you should actually survive the attempt of killing Morgoth, you will receive the status of WINNER. You may continue to explore, and may even save the game and play more later, but since you have defeated the toughest creature alive, there is really not much point. Unless you wish to listen to the rumors of a powerful ring buried somewhere in the dungeon, or a suit of dragon scale mail that resists everything…
When you are ready to retire, simply kill your character (using the Q key) to have your character entered into the high score list as a winner. Note that until you retire, you can still be killed, so you may want to retire before wandering into yet another horde of greater demons.