Could someone please check my sentence if it is ok?

It's a bit awkward.



Try
Yesterday I got a letter from a stranger, telling me to transfer a large sum of money into his account by 6am tomorrow if I don't want to see my beloved friend killed.
or...if I don't want my good friend to be killed.
or...if I want to prevent my best friend being killed.

Thank you very much.

...a letter saying......unless I want.......

Thank you.
By the way, does it matters much with and without 'in which'?

If you want to use "in which" you can say:

Yesterday I recieved a letter in which someone unknown said that I have to transfer a large sum of money into his account by 6am tomorrow if I don't want to see my belovend friend killed.

Thank you.

What are you trying to ask? Can you explain.....