Death Gate is a death knight ability that allows the death knight to open a gate to Ebon Hold on a 1-minute cooldown.
The Death Gate appears to be an actual physical gateway, similar to a mage's portal. All death knights in the same party may use a portal created by any of them. The portal does not work for any other class.
This ability cannot be cast during combat, but an existing death gate can be used during combat. It may be planned preemptively to use this as a "just in case" getaway when attempting something difficult for your level.