Conversation so far:
What was Helloween?
the German power metal band.
When did he join?
At the age of eighteen, Kiske was asked to join.
What did he do in the band?
vocalist.
Are there any other interesting aspects about this article?
Kai Hansen left the band and was replaced by guitarist Roland Grapow.

Question: When did he leave?
Self-contained rewrite: When did Michael Kiske leave Helloween?