A man returns to the castle where his archaeologist parents were murdered when he was a boy. He brings his girlfriend and 2 other friends. He starts to remember that he killed his parents with an immortal boy king who can’t leave the castle. They then kill his 2 friends and try to kill his girlfriend. I think she escapes.
The Playroom (AKA Schizo) (1990)
https://www.imdb.com/title/tt0100373/?ref_=nv_sr_srsg_2_tt_8_nm_0_in_0_q_the%2520playroom
That’s it! Thank you – this has been driving me mad for years.
Solved!