At level 510 the Undead Pirates will begin to appear on your Battle List, and are great for fighting at higher levels when living opponents begin to seem fewer and far between. You'll be able to tell the apart on your Battle List as they'll have purple names and a small tombstone beside them.

Unlike a normal Pirate, they don't carry Gold and can't be hurt. Undead Pirates will only appear on your list when you've defeated all the regular Pirates, or there aren't many available. If your list is still full of Pirates for you to fight, the system won't generate Undead ones for you. Keep in mind you can only fight them so many times in a day before they melt back into the shadows and disappear, but not before you get your max daily Gamer Points!