How do I rotate an existing vector by a relative yaw.

I am new to the engine still. Also it’s been a while since my last math course. Basically what I’m looking to do is run a line trace exactly behind the player and downward 45 deg. I tried rotating the inverse of the forward vector from the FirstPersonCharacter by 45 deg on the x axis and that works fine at spawn when the forward vector has a y value of 0. But when you look to the left or right, it obviously isn’t working because there needs to be some sort of y rotation. How do I do this?

(another explanation/way to think of problem) - I want take the backward vector from player (character not cam) and rotate it relative to the character by 45 deg along the x-axis. I then want to use that vector in a line trace so I think it needs to be in World coords.

Line trace would look like this -start = Player Character, -end = Player Character + backward vector rotated down 45 deg.

Thanks for all the help already found in the forums great community,
TMGL16