The Immortal (achievement)

The Immortal is an achievement earned for killing all of the bosses in Naxxramas without any members dying during the Wrath of the Lich King expansion.

This achievement is no longer criteria for the meta-achievement, Glory of the Raider (25 player). Players that completed this achievement retain the title associated with it.

Q & A
Bornakk answered some questions about the achievement on the forums: