The Amulet of Yendor is a , character:
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Once you get it, you can travel up stairs using <.
There will be no items on any floor you've already been to. So once you start going up, any item you see is a Xeroc in disguise.
The later dungeon levels contain monsters at you that you cannot beat by combat alone. Dragons, Griffins, and Jabberwocks cannot be faced head-on. Dark unicorns and Medusae are very risky as well.
Luckily, you have a lot of other options. Here are some of the best.
Scroll of Scare Monster - Drop it on the ground, and you cannot be hit while standing on that square. This lasts until you pick the scroll back up, at which point it disintegrates. If you walk off the square, you can use the m command (move) to move onto it without picking it up. Great when you encounter a room full of monsters.
Staff of Polymorph - Replaces the monster with another random one. Usually this just solves your problem outright.
Scroll of Hold Monsters - Gives a few turns for you to get away, or reach stairs that are blocked by monsters.
Scroll of Random Teleport - Gets you out of combat immediately. Hopefully to a better place.
Wand of Cancellation - Better than it sounds! Removes a monster's ability, so you don't have to risk losing levels to the Wight or getting hit with the Medusa's gaze. Often comes with many charges.
Wand of Drain Life / Fire / Magic Missile - Lots of damage at range. You can kill griffins and even dragons with these.
Healing Potion - Gives you another few combat rounds. Most useful on earlier monsters like trolls and centaurs. Late game monsters do so much damage that the heal potion barely gains you one round.
Ring of Regeneration - Put it on before combat to give yourself the option of healing by running.
Running away - Very risky, as you may run into another monster and become trapped. But if you can reach a branched tunnel on the map, your pursuers will give up and wander off.
Try to carry at least 2 food items at all times. Don't eat until after you've been "weak" for a while.
Wearing rings speeds up hunger, so use rings sparingly.
As long as you're not wandering aimlessly, you should have enough food.
Once you have the Amulet of Yendor and begin ascending, your digestion slows. Having 1 food on hand and not being hungry should be enough to reach the exit on the first floor.
Type Ctrl+W and you will be prompted to enter a password. The password is bathtub. This puts you in Wizard Mode. In Wizard Mode, all your items are identified. You can freely move between floors as if you had the Amulet and without the need for stairs. Wizards take less damage from attacks, dodge more, and deal more damage.
If you have ever been a wizard, you don't get onto the high score board when winning the game.