This is the letter which is wtitten in S
I will open the red box.
That is the thief who is tied to a post by a rope.
They are doctors who lack kindness.
They are slaves whom I own.
She is a slave whom I own.
He is a doctor who lacks kindness.
All correct but not natural because a single sentence will do.
I gave John the cat which I got (received) as a present.....etc.
You need to learn about adjectival clauses.