If cell nextdoor is ___, return the value of a relative cell

Can't figure this out.

I want to say
"Find the value of the cell next to this in a separate list, and return the value to the left of that"

I would fill it down to follow this pattern
In cell A6, I want to find B6's content in a list of $D$10:$D$20, and whichever cell it is found in (ie D12), return the cell to the left (C12).
In cell A7, I want to find B7's content in a list of $D$10:$D$20, and whichever cell it is found in (ie D15), return the cell to the left (C15).

I guess it would help if there's a way to say "this cell's address" in order to say "1 right of this".