The Shadow Brand Skill is a new skill unique to Celembrior for the Bright Lord dlc. The skill is nearly identical to Shadow Strike, except it requires three elf-shot to use instead of two and any uruk hit with it gets branded. Like Shadow Strike you can chain Shadow Brand.
To use it simply hit
to bring up your bow, aim at an uruk or your choosing, and then tap
to brand it. Simply repeat 19 more times. You'll get this quite easily over the course of the dlc campaign.