Banishment
Banishment refers to sending a monster or the player to the Abyss. Although it causes no damage, a trip to the Abyss is often fatal for low- or mid-level characters. There are numerous ways banishment can occur:
- Certain monsters have the ability to banish the player via a LOS-range, beam-targeted spell. This can be resisted with high magic resistance.
- Being hit by a branded weapon of distortion can cause banishment (10% chance). For the same reason, being hit by a chaos-branded weapon will occasionally cause banishment. Unwielding a weapon of distortion poses an even greater risk (25% chance).
- Banishment is a severe Translocations or Summoning spell miscast effect.
- An attempt to vorpalize a temporarily branded chaos or distortion weapon can result in banishment.
- Zot traps cause random magical effects, and can occasionally cause banishment.
- Lugonu worshipers can banish monsters via Lugonu's "Banishment" ability. Similar to the monster spell, this can also be resisted. Worshipers can also banish themselves via Lugonu's "Gateway yourself to the Abyss" ability; this is meant as an emergency move.
- While the player is under penance, the wrath of Lugonu, Vehumet, Nemelex Xobeh, Makhleb or Xom can result in banishment. Xom worshipers may also be sent there if he becomes bored.
- The Damnation card banishes random creatures in your area (and possibly the player). The Xom card has the potential to banish you.
Banishing sends a monster and everything it's carrying to the Abyss (try not to banish monsters that are carrying items you want!). You gain partial experience based on the percentage of its remaining HP. While normal enemies are gone forever, banished unique and powerful monsters will be found wandering the Abyss. Likewise, any artefacts banished to the Abyss will eventually generate there. Being banished while in the Abyss will send you to a deeper level of the Abyss, where you'll find stronger monsters (but also better loot, possibly including the Abyssal Rune).
Magic resistance helps against the monster spell and Lugonu invocation only. It does not help against any of the other means of banishment.
When leaving the Abyss after being banished, you will return to the Dungeon level, floor of Pandemonium, or Ziggurat level that you were formerly on. Note that this is different from leaving the Abyss after you voluntarily entered it with a portal; in that case, you will return through the same portal you entered (unless it was a portal in Pandemonium, in which case you'll return to the main Dungeon).
Monster Banishment
By Ranged Magic
You need higher magic resistance to resist more powerful spellcasters' attempts to banish you. Generally, magic resistance that is well into "extremely resistant" will be enough to resist even draconian shifters and ancient liches.
Banishment can be cast by the following monsters if you are in their LOS:
@ 
Louise
e 
Deep elf sorcerer
e 
Deep elf demonologist
O 
Erolcha(depends on spell set)
O 
Ogre mage(depends on spell set)
e 
Deep elf mage(depends on spell set)
L 
Lich(depends on spell set)
p 
Wizard (monster)(depends on spell set)
L 
Ancient lich(depends on spell set)
& 
Pandemonium lord(depends on spell set)
By Branded Weapon Attack
These monsters may generate with a distortion- or chaos-branded weapon. A hit by a distortion weapon has a 10% chance of Banishment, while a hit by a chaos weapon has roughly a 1% chance. Additionally, any monster listed below without a specific weapon type may spawn with a polearm and thus have the capability of a reaching distortion/chaos attack. Monsters are listed with the weapon's brand and percentage chance of it possessing such a weapon:
g 
Crazy Yiuf(quarterstaff of chaos, 100% chance of possession)
@ 
Psyche(dagger of distortion, 25%; dagger of chaos, 75%)
K 
Sonja(dagger of distortion, 50%)
@ 
Donald(distortion, 4%)
@ 
Maud(distortion, 4%)
@ 
Frederick(distortion, 4%)
@ 
Margery(distortion, 4%)
p 
Hell knight(distortion, 4%)
i 
Spriggan defender(distortion, 10%)
i 
The Enchantress(distortion, 25%)
Note that any other monster capable of wielding weapons can pick up a distortion weapon and use it. This can result in even lowly monsters such as goblins banishing you, albeit very rarely.
By Melee Attack
These monsters have a chaos-branded attack, which can (rarely) result in Banishment:
4 
Chaos spawn(also trails clouds of chaos)
t 
Apocalypse crab
w 
Eldritch tentacle(by way of a monster-initiated Malign Gateway)
Additionally, some monsters have an innately distortion-branded attack:
v 
Spatial vortex
v 
Spatial maelstrom
Note that pandemonium lords and player ghosts can have chaos- or distortion-branded melee attacks.
History
Prior to 0.8, there was a resistible player version of this spell which sent one target to the Abyss. It was a level 4 Translocations spell. Renouncing Lugonu, Makhleb or Vehumet could result in immediate banishment.
Prior to 0.9, one of the possible outcomes of Hell's mystical force was banishment.
Prior to 0.11, self-banishment was a popular way to leave Ziggurat floors that you just couldn't clear.
Prior to 0.12, being banished while in the Abyss had no effect.
Prior to 0.13, draconian shifters were able to banish you.
