From Guild Wars 2 Wiki
Grace Period is an effect applied in Southsun Survival during the first three minutes of the match, preventing permanent death. Any player that dies after its expiration will be turned into a Mist Walker.“
Grace period is in effect. Upon death, you will be teleported to a random camp and revived. After the grace period is over, you will fade to the Mists when killed.
— In-game description