A Banishment, also known as an Imprisonment, sends a fellow to another Plane. How they get back to the regular Plane is up to them.

Most Banishing occurs in combat. The common destination Planes for a Banishment are the Inferno, Kokytos, and the Void.

• A fellow may be banished to the Inferno by Infernal Prison, a 31 Blood spell of the 8th level.
• A fellow may be banished to Kokytos by Claws of Kokytos, a 31 Blood spell of the same caliber as Infernal Prison.
• A fellow may accidentally banish themselves to the Void with the Returning spell, a 2 Thaumaturgy of the 2nd level, though the spell is meant to teleport the caster to their nation's capitol. Likewise, the Vortex of Returning spell (4, Thaumaturgy 7) has a small chance to banish any of the caster's allies instead of teleporting them.

Note that Banish Demon isn't really a Banishing spell, but rather an instakill for Demons.

There are also Artifacts that do the job, on hit, though sometimes that's easier said than done.

Technically speaking, Banishments are forced teleports to specific provinces that have no province connections, internally called "Planeshifts". With Dominions 6's addition of Nexus to the Void, the definition has gotten a bit fuzzier; it isn't yet clear if being Banished to the Void can send you there, however. Internally, the Inferno is considered Province -12, and Kokytos is considered Province -13.

Banishment is effectively the same thing as an unsuccessful rout for non-commanders, since they end their turn in a foreign land without any leadership; in other words, Banished troops are effectively killed. Commanders can survive in the other Planes, and have a small chance to make it back to the regular Plane each turn:

• For the Inferno and Kokytos, the base chance is 8% each turn, though with only a 50% chance to end up in a friendly province.
• For the Void, the base chance is 5% each turn, and the chance to end up in a friendly province is determined by your Astral level; 10%, plus 20% per Astral level.

Note that several items provide an increased chance to escape specific Planes; Lifelong Protection gives adds 10% to your chance to leave the Inferno, for example, while a Lightless Lantern adds 10% to your chance to leave the Void.

Units cannot resist a Banishment spell, but they can kill their would-be-Banishers outside of the spells' 35-square range. Alternatively, the spells' 2-and-200-Fatigue cost can pose an issue once the casters can finally get in range; the former can be whittled down ahead of time, while the latter can cause physical damage to the caster if they've done any spellcasting beforehand. With all that said, the Banishment spells work wonders for winning against "thugs", "supercombatants", and Idol-type Pretender Gods who just won't die.

There are no Banishment Rituals. Banishment can still happen out of combat, but is typically the result of bad random events and/or poor planning. Most of these events have a certain condition that is unclear in the Dom6 Mod Inspector; either 4 or higher, or 4 levels in any Magic path, or at least 4 levels total.

• The most reliable way to Banish your own commanders outside of battle is with the Void Gate, which Middle and Late R'lyeh have. This has a small chance to send Astral mages that use it to the Void.
• If you have 2 on the local Scales, demolishing a Laboratory has a chance to Banish the demolisher into the Void.
• A number of unique summons from Blood Magic can get annoyed or creative with commanders in the same province, resulting in a commander there being Banished. These include five of the Ice Devils, three of the Heliophagi, Buriol - Arch Devil, and Buer - Goat Sun.
• The Looming Hell adds the possibility for commanders to randomly "disappear", or to "strike a deal with Infernal powers", though many other events are made more likely during this Global Enchantment. In these events, the Banished fellow is guaranteed a pathboost from their Banishment; 1 if they got sent to the Void, 1 if they got sent to Kokytos, and either 1 or 1* if they got sent to the Inferno.
• If you have Story Events enabled, a Demon Cult event chain can start in lands of Order, which can also lead to The Looming Hell's Banishment events.

*Might be 1, or might have been that before Dominions 6.

Remaining Banished for any extended period is not ideal:

• Each turn that you're in the Inferno, you take 1D20 AN Fire DMG. The Heat from Hell Battlefield Enchantment is always active here. Each turn, you have a 50% chance to be mobbed by either 2D30 Imps or 1D8 Devils. On the bright side, if you manage to get out and you don't have 4, you have a 10% chance to gain +1 from your experience.
• Each turn that you're in Kokytos, you take 1D20 AN Cold DMG. The Grip of Winter Battlefield Enchantment is always active here. Each turn, you have a 75% chance to be mobbed by either 2D10 Unfrozen, 1D20 Winter Wolf fellas, or 1D8 Frost Fiends. On the bright side, if you manage to get out and you don't have 4, you have a 10% chance to gain +1.
• Each turn that you're in the Void, you gain 1D6 Insanity; which doesn't matter much while you're in the Void, but becomes a problem outside. Those with Void Sanity on their character sheet divide all their Insanity gains by their point value of the trait, though. Fighting in the Void is terrible without Void Sanity on your sheet, too. Each turn, you have a 15% chance to be mogged by various abominations. Getting out with less than 4 has a 30% chance to give you +1, and 2D50 Insanity.

These rolls are all open-ended (hence the capitalization of the "D"), meaning their results can be above the typical maximums.