Alignment key (dNethack)
dNethack adds alignment quests/keys similar to Slash'Em, but substantially more complicated. Each quest contains 3 keys instead of one, and almost all of them have a boss you need to fight. You need 3 keys to open the doors to Gehennom, so it's not necessary to gather all of them.
The First Key of Law
The First Key of Law is found in the Dungeon of Ill-Regard, and is guarded by Axus. He has a powerful stunning reflexive melee attack, so it's best to kill him from a distance if possible. He is _not_ magic resistant, so a wand of death will work on him. After you kill him, he'll revive in a couple turns, and eating his corpse is fatal. Axus (and the autons on the level) is usually generated peaceful for lawful characters.
The Second Key of Law
The Etched Second Key of Law is found in Arcadia, and is guarded by Arsenal. He throws iron balls at you (2d6), showers you with magic missiles (2d6), and has a powerful kick (8d2). He is a form of golem (or at least shares the glyph), and is resistant to death magic and all major elemental damage, though with decent AC and magic resistance he shouldn't pose a threat.
The Third Key of Law
The Third Key of Law is found in the Arcadian Tower, and is guarded by Lady Oona. She has 2 melee physical attacks (2d8 each), but the real kicker is an active 16d8 elemental damage spell, and passive elemental damage. Her affinity is chosen randomly each game, from fire, cold, and electricity. Some players opt not to kill her, and instead tame her. She makes a powerful pet, and is humanoid and thus can wear most kinds of armor.
The First Key of Neutrality
The First Key of Neutrality is guarded by Center of All. He can be found anywhere in the main dungeon, appearing as part of standard monster generation (albeit with an extremely low chance). However, the bottom level of the Outlands has a greatly increased chance for him to spawn.
The Second/Third Key of Neutrality
Alhoons are the guardians of the Second and Third Keys of Neutrality. They are not unique like the other guardians, but a type of monster. However, only the 2 guardian alhoons ever spawn. They are a form of undead master mind flayer, turned into a lich. They have a cold-based touch attack, like standard liches, intelligence-draining tentacles, and can cast monster mage spells.
The first alhoon is on the Gulf of N'kai (3 levels under Sum of All, where Center of All often spawns). The second is at the temple complex at R'lyeh, a level or two below that.
The First Key of Chaos
The First Key of Chaos does not have a guardian that will drop it on death. It's well-hidden instead. If you don't want to figure it out yourself, please consult the First Key of Chaos spoiler page.
The Second Key of Chaos
The Etched Second Key of Chaos doesn't have a guardian either. It's randomly placed somewhere in the Earth Temple. It can be in either the outside or inside section. You may want to deal with Lich, Fiend of Earth before trying to find it however.
The Third Key of Chaos
Chaos himself has the Third Key of Chaos. Unless you really need it, or are already planning to get The Black Crystal, then you may want to get a different alignment key to open the doors instead. He's a powerful spellcaster, able to heal himself, summon high-level &, and cast other monster spells. He's at the Chaos Temple, at the very end of the other elemental temples. When he is killed, you are also able to pick up The Black Crystal (which grants slotless MR).