The Eidolon Plane is the destination of characters who die (in combat or otherwise). It is inhabited only by the Eidolon being.
If playing in Adventure mode and still having any lives left, or Exploration mode, you are transported here and given the chance to rest, return to the same location, (if you died in the Infinite Dungeon) be returned to a random location on the same level, (if you died elsewhere) be returned to the world map outside the zone, or to give up for good (your character is dead for good).
This zone appears to be somewhere in space, and there is no gravity in the zone. It can not be left except by the Eidolon's choices.
You cannot autoexplore in this zone. No chronomancy anomalies may occur here. You cannot drop any items here - they will return to your inventory.
If you die on the Eidolon Plane, it is permanent, and you recieve the achievement Utterly Destroyed.
When using the Exploratory Farportal in the Sher'Tul Fortress on Roguelike mode, there is a 10% chance to be sent to the Eidolon Plane (only once, and only 9.5% if you haven't been sent to Caldizar's Fortress yet). This is the only way to reach the Eidolon Plane on Roguelike mode, and therefore killing yourself here is the only way to recieve Utterly Destroyed. If you talk to the Eidolon on Rougelike mode, it will force you to leave immediately, so do not talk to it if you want the achievement.