Quote Originally Posted by KrayZ33 View Post
I believe they said that once berserker goes into 'insane' mode, he is unable to get out of it again... normally
he keeps draining his master's mana which results in certain death.
From the apparently questionable Typemoon wikia

Berserker: Servants placed within this class are always incarnations of heroes who have gone berserk on their lifetime. This trait allows them to use the special ability Mad Enhancement, which trades their consciousness (i.e. sanity) for a large power boost. Most Masters are incapable of controlling their Servant once Mad Enhancement has been activated, which eventually results in their death.