The following message was posted on the official Bloodbath website:

Mike Åkerfeldt - former Bloodbath member with possibly the best death metal voice in the world has stepped up to help the band destroy Wacken on this exclusive one time happening. This is something that you're guaranteed to never see again as Mike will not be rejoining the band in any way apart from committing to this performance

In related news, Peter Tägtgren has left the band due to conflicting schedules. We thank him for sharing his acclaimed talent and efforts on "Nightmares Made Flesh." Bloodbath live at Wacken, Blackstage, Friday August 4th:

Mike Åkerfeldt - Vocals
Blakkheim - Guitar
Dan Swanö - Guitar
Jonas Renkse - Bass
Axe - Drums

Source:metalunderground.com