Board Thread:Questions and Answers/@comment-100.0.91.117-20150217040429/@comment-25669516-20150217093030

I think it's the "reach level 40" achievement.

Same thing happened with me. I bought the Witch Impaler but still did not acheive the corresponding Achievement ( I think it was "Witch Will Cry" or something ) for buying the weapon. Probably some bug...