[spoiler]:/ it's complicated. Long story, but they are small race fewer than 100. Not much is known of them. Dolph caught one and diced it up, but another came through and destroyed this planet and brought the "dead" Immortal back to life. Which is why Dolph wants another one. That said each Immortal has a special ability only was capable of bringing back the dead and has never been heard from since.[/spoiler]