Special rooms are a unique kind of vault. They are one-tile-border rectangular rooms of variable size (6 to 9 by 5 to 9) stuffed with up to 27 monsters, with tiny flat chances per valid branch and given layout. Multiple can appear per game (for most sets), unlike normal vaults, and their chances are unaffected by the number of mini-vaults implemented for a given branch. Many early roguelikes also have these styles of vault.
Special rooms can spawn in the Dungeon, the Lair of Beasts, the Spider Nest, the Elven Halls, the Crypt, the Depths, and the non-end floors of the Orcish Mines, the Vaults, the Slime Pits, Gehenna, Cocytus, Tartarus, and the Iron City of Dis. Monster sets have explicit, coarse depth-scaling, so it is quite rare to find (for example) a Jelly Pit on Lair:8 in the first place. Most special rooms have some kind of meager items or mildly useful dungeon features built in as a reward. Most special rooms also have a chance for a boss, which can vary from simply guaranteeing the placement of the top of the depth-scaling range, to adding thematic further threats.
One of the two sets that cannot repeat per-game.
Monsters: Kobolds and kobold brigands. The code contains further coarse scaling than is currently used, but still scales rather simply as follows:
|K Kobold brigand||50%||60%|
The boss monster is always a kobold brigand on D:7-11, but on D:12 and in Orc:1-3 there is a coinflip chance between it and the following:
Monsters: Orcs, alongside the occasional ogre or troll as seen in the Orcish Mines.
|Depth||D:9||D:10-14||Orc||D:15, Vaults:1||Elf, Vaults:2-4|
|o Orc warrior||10%||20%||25%||25%||33.33%|
|o Orc priest||0%||4%||10%||10%||13.33%|
|o Orc knight||0%||5%||15%||26.67%|
|o Orc high priest||0%||3.33%|
|Boss||D:9||D:10-14||Orc||D:15, Vaults:1||Elf, Vaults:2-4|
|o Orc warrior||66.67%||10%||0%|
|o Orc priest||33.33%||70%||0%|
|o Orc knight||0%||20%||40%||50%||0%|
|o Orc sorcerer||0%||4%||0%||0%|
|o Orc high priest||36%||45%||10%|
|o Orc warlord||5%||20%||90%|
Rewards: There is a 11.11% chance per square for every square with a non-boss monster to have depth-adjusted low amounts of gold on them, and the boss monster always stands on one regular slightly-level-adjusted pile of gold. On a coinflip chance, said boss monster also stands on an altar of Beogh.
One of the two sets that cannot repeat per-game. Special rooms in Spider, which are always bee hives, have their walls randomly replaced with floor.
Monsters: By far the simplest of special_rooms, containing one of the second and filling the room with the first:
Monsters: Extremely varied spread of undead and related monsters, from derived undead to demons. It is worth noting that the Hells will specify the weapon qualities of the skeletal warriors, vampire knights, and reapers: flaming brand for Gehenna, freezing brand for Cocytus, draining brand for Tartarus, and any brand as is valid for the weapon in Dis. Said skeletal warriors and vampire knights are also exclusively equipped with either a great mace, a glaive, or a battleaxe.
|Monster||Depths chance||Crypt / Hells chance|
|W Spectral thing||1.43%||1.25%|
|W Shadow wraith||2.86%||2.5%|
|3 Soul eater||2.86%||2.5%|
|W Flayed ghost||2.86%||2.5%|
|z Skeletal warrior||21.43%||18.75%|
|V Vampire knight||7.14%||6.25%|
|D Bone dragon||1.25%|
|Branch specifics (collectively)||28.57%||25.00%|
Said branch-specific monsters:
|e Deep elf death mage||7.69%|
|D Shadow dragon||7.69%|
|Crypt / Tartarus||W Eidolon||8.11%|
|n Plague shambler||2.56%|
|M Mummy priest||2.56%|
|q Deep dwarf death knight||2.56%|
|Gehenna||Z Z Fire giant skeleton / zombie||8.11%|
|Z Z Fire dragon skeleton / zombie||5.26%|
|z Flaming corpse||6.67%|
|W (extra) Flayed ghost||6.67%|
|Cocytus||Z Z Hydra zombie / simulacrum||8.11%|
|Z Z Alligator snapping turtle skeleton / zombie||5.26%|
|W Freezing wraith||9.59%|
|n Bog body||3.9%|
|The Iron City of Dis||Z Z Iron dragon skeleton / zombie||8.11%|
|Z Z Iron troll skeleton / zombie||5.26%|
|z Ancient champion||9.59%|
|W Phantasmal warrior||3.9%|
The boss monster is a lich or, if in Depths and Crypt and passing another depth-scaled chance, a vampire mage:
It is also worth noting that every branch has vaguely thematic corpses and skeletons littered across the vault, with each square having a 40% chance to place a corpse (outside of the Depths, Crypt, and Tartarus, where it is a 33% chance). Corpses are of valid branch monsters for the Depths, elementally-related giants / trolls and dragons for Gehenna / Cocytus / Dis, and a mix of shadow dragons, base draconians, and dire elephants for Tartarus / Crypt. Of course, this allows monsters within the vault to cast Animate Dead and its derivatives.
Rewards: In Depths or Crypt, the boss stands on an artifact spellbook of Necromancy. Within the Hells, one monster is replace with a 60% chance for a downward staircase (if no other vaults on the floor have required staircases, which will randomly split the floor's staircases across said vaults if all staircases are thus in-use), a 20% chance for a downward escape hatch, and a 20% chance for empty floor.
Special rooms in Slime, which are solely jelly pits, have their walls randomly replaced with floor.
Monsters: Jellies of all kinds, bar pulsating lumps and slime creatures. There is some absolute-depth scaling for monster placement, amplified for Slime: this table covers the earliest, latest, and the most commonly-seen stretch for the floors to place. For intermediate values, Slime:1 is here equal to Depths:4.
|J Brown ooze||30.67%||32.94%||34.78%||35.92%|
|J Death ooze||16.00%||18.82%||19.57%||21.36%|
|J Azure jelly||2.67%||4.71%||5.43%||6.80%|
|J Acid blob||1.33%||2.35%||3.26%||4.85%|
Rewards: If the vault has placed in Depths or Lair:8, there is a coinflip chance for an altar to Jiyva to be placed within the vault, randomly replacing a monster or floor tile.
Monsters: "Standard mythological beasts", according to the commit adding them in. There is very fine absolute-depth scaling involved in the monster weighting, further special-cased for Lair and Elf; this table covers the weakest scaling and strongest scaling on either side of the threshold in Elf / Depths. As the table states, said threshold contains harpies, fire dragons, and sphinxes only on the deeper side. For intermediate values: D:14's chances are equivalent to Lair:6, and Depths:1 chances are equivalent to Elf:2.
|k Fire drake||20.59%||11.32%||1.35%||0.71%|
|h Hell hound||1.47%||9.43%||13.51%||11.43%|
|D Fire dragon||6.76%||12.14%|
Boss monsters have more coarse depth scaling:
|Chances||Lair:4-7, D:14-15||Lair:8||Elf, Depths|
|D Fire dragon||0%||38.89%|
|2 Hell beast||0%||22.22%|
Rewards: Underneath the boss monster is a small or large pile of gold, with chances for either scaling slightly on depth: a 79.16% / 20.83% respectively on Lair:4, and a 62.96% / 37.04% chance respectively on Depths:5. The former slightly scales with depth as a normal gold pile, while the latter is fixed (with even chances) for 100 to 499 pieces. There is also a 15% chance for a box of beasts.
- 0.9-5b059c0c excluded non-morgue special rooms from Gehenna, Cocytus, and Tartarus.
- 0.11-1deeae5e capped special room contents at 30, replacing what previously at their absolute most was 121 of their given monster set.
- 0.12-6cbb9191 contained the majority of recent changes to special rooms. Beyond adding the mythical zoo set, all but the jelly pit contained little to no depth scaling, vastly weaker sets, and much farther ranges. All but the kobold and bee rooms also lacked monsters, and all but the bee rooms contained no explicit items or features. Rooms were also restricted to one layout (but more common within that layout), slightly larger, had a door in their entrances, and gained a lower cap of 27.
- 0.14-188663e2 changed the morgue monster set substantially to reflect the Crypt overhaul in 0.13.
- The dungeon split inherent in the Depths in 0.14 further polarized the depth-scaling calculations to be as is visible now.
- In 0.15, with the removal of brown oozes, jelly pits instead contain great orbs of eyes. The removal of plague shamblers was not accounted for in the odds balancing for morgues, but the removal of flaming corpses moved the extra weight further onto the extra flayed ghosts.