Angband

Angband is a roguelike game loosely based on JRR Tolkien's books set in Middle-earth, but with stuff from other games, mostly Dungeons and Dragons and Role Master, as well as some original content. It was intended as an enhanced port of Umoria, an earlier roguelike. The goal of the game is to descend into the dungeon and amass enough experience and loot to defeat Morgoth, the Big Bad of Middle-earth.

Not as well-known as Nethack, but has a comparable fanbase. It is relatively easy to alter the source, so it has spawned hundreds of variants over the years, a few dozen of which are still in active development: one example is Z Angband. The latest version of Angband can be downloaded at http://rephial.org/.

Tropes in this game:

 * Cast From Hit Points: Genocide/Banishment spell, which will subtract 1d3 HP for every monster killed from the spell. Also, casting a spell without enough mana could damage your CON (health stat) temporarily or permanently.
 * Chest Monster: Creeping Coins and various other mimics
 * Companion Cube: Slime molds, a food item which many players name and keep as pets
 * Death of a Thousand Cuts: Explosive breeders. Most dangerous for low-level characters, but a sufficient number of them can cause problems even for higher-level players. If met at the start of the game, a Floating Eye can keep you paralyzed to the point that you starve to death.
 * Drop the Hammer: Priests are restricted to using bashing weapons. Also, Morgoth's hammer Grond, which shatters walls all around when swung.
 * Eldritch Abomination: Quylthulgs. Cthangband is a variant entirely based on the Cosmic Horror Story genre.
 * Enemy Summoner: Better learn to make an anti-summoning corridor.
 * Evil Is Deathly Cold: Cursed items
 * Explosive Breeder: Many low-level creatures, including worm masses, lice, and rats, each of which which have many annoying colour variants. These can be ground upon to gain levels, but unless you're using a borg (a bot which plays the game) this is too dull to do for more than a few levels.
 * Fantasy Gun Control: A few variants avert this, notably Steamband.
 * Final Death: It's a roguelike. You can savescum, but don't try to pass it off as a genuine win.
 * Gateless Ghetto: Built on top of Morgoth's fortress is a town consisting of six stores, a black market, a house belonging to the player, and a few ne'er-do-well townfolk, surrounded by an impenetrable wall. A few variants avert this by adding an overworld.
 * Guide Dang It: Don't go below 1000 feet (20th floor) without See Invisible. Don't go below 2000 feet (40th floor) without Resist Poison.
 * Loads and Loads of Races: Mostly Z Angband. Steamband also does this, but many of them are different nationalities of human.
 * The Z Angband variant Entroband has thirty-seven races and twenty-seven classes.
 * Random Number God: Everything in the dungeon is randomly created. Surviving early on depends entirely on random luck.
 * Schmuck Bait: Greater Vaults. If you see a completely walled off room filled with treasure, run away.
 * Also, in To Me, don't equip that yellow ~ lying on the ground right away. It could be the Phial of Galadriel, or something really evil...
 * Stat Sticks: Defender weapons.
 * Vendor Trash: Unidentified potions of apple juice, slime mold juice and water, which are given the same color qualifier across games (although no longer available in the latest versions). You can also sell other unidentified items as a cheap identification method. However, known trashy items cannot be sold.
 * Video Game Cruelty Potential: The town level is full of beggars, harmless drunks, street urchins and mercenaries. The urchins of whom will try and steal from you. Guess what happens next. Even worse, the mercenaries can easily kill a starting character.
 * Three words: Killing Farmer Maggot.
 * Sure, but that's not purely for fun, he usually drops good stuff.
 * In Z Angband, you can kill the Finnish president and Nobel Peace prize laureate Martti Ahtisaari - or rather, a parody version of him.
 * A fairly minor but entertaining example is selling broken swords/daggers and rusted chain mail to shops. The storekeeper will offer you one gold coin for it and then - surprise, surprise - it turns out said item has negative to hit/to dam/armor modifiers, thus making them worthless.
 * Warp Whistle: Scrolls of Recall. Also available as a cleric spell.
 * Weapon of X-Slaying: One of many properties ego and artifact weapons can have is bonus damage against particular types of enemies. This is controlled by a set of independent boolean flags, allowing (in the extreme) the artifact sword Crisdurian, which has all the Slay X flags set (and has the damage dice to make it count).
 * Yet Another Stupid Death: It's a roguelike. This is inevitable.