Dead Cells

You will acquire the Servant outfit blueprint when you kill the final mini-boss in the Lighthouse, IF you have already acquired their weapon.
For example, if on your first visit to the Lighthouse you kill the archer last, you will get their bow. If on your second run you kill the archer last again, you'll get the outfit. It's a reasonably expensive outfit so you'll probably need to do another run for the cells to unlock it, then another run after that with the outfit equipped. Unfortunately it doesn't let you stealthily infiltrate as the achievement name might suggest and you get attacked as normal!