Advent Void

The Advent Void is the alternate dimension where Kamen Riders are sent after losing a battle, simply called "Getting Vented", in the Kamen Rider Dragon Knight series. It is used as an alternative to the principle that defeated Riders are "eaten" by their opponent's Contract Beast that was used in Dragon Knight's original Japanese counterpart, Kamen Rider Ryuki. Instead of dying, the defeated Rider is sent to the Advent Void after taking massive amounts of damage as a safety mechanism to ensure that the Rider isn't killed.

Trivia

 * Len refers to the Advent Void as "the world between the Mirrors" implying that it is the empty white space that the Kamen Riders traverse to get to and from Ventara, but when Eubulon retrieves the original Axe, Strike, and Torque the Advent Void is a multicolored space containing pods with Vented Kamen Riders asleep inside them.