Skithiryx, the Blight Dragon

If the regeneration shield is used, Skithiryx isn't destroyed, all damage marked on Skithiryx is healed, it becomes tapped, and it's removed from combat (if applicable).

Activating the regeneration ability creates a replacement effect that acts like a shield, replacing the next time Skithiryx would be destroyed that turn. The shield works against effects that try to destroy Skithiryx or lethal damage that would be dealt to Skithiryx.

Skithiryx can regenerate even if it isn't in combat, it's already tapped, or it's undamaged.