Challenge the Devil (1963) ★★★ 
That was a weird one, it starts off as a spy thriller with some story about extortion/blackmail and then some guy describes the thing that happened to him in a castle with his friends that made him become a monk... for an
hour-long flashback.
Christopher Lee appears for around 10 mins as an old man in a castle, who turns into a young man in a castle with very hairy hands.
There is also an extended nightclub scene, like a cross between a musical interlude from a Japanese gangster flick made by Nikkatsu and something Jess Franco would give you in a lot of his movies, only it keeps focus on two couples that aren't even part of the narrative (but one does the best 'not impressed' face at the 3 acts we see).
We also get a full on existential crisis with men screaming and crying and panes of glass being smashed with candelabras and a large spider who's spying on our group of young delinquents.
I quite liked it!