Pretty straight forward achievement, just complete Act 5 with the Necromancer.
I managed to unlock this by joining a friends game, in story mode and used the teleport pad to Pandemonium fortress, progressed through to Malthael (Final boss) killed him and then unlocked the achievement.
I can't confirm whether it will also unlock by killing Malthael in adventure mode or not. If anyone can confirm/deny in the comments i'll update the solution.
Edit: Comments would suggest that it does not unlock in adventure mode