In Shadow of Mordor, you play an angry man haunted by a sad elf. The Bright Lord DLC tells the story of how that elf became sad, and lets you use The One Ring to take it out on Sauron. It plays out like a miniature version of the main game, which means you'll be magically turning orc generals to your side to get closer to the final boss. The difficulty has been hiked, so don't expect quite the same levels of wanton slaughter enabled by the second half of the main game.
Celebrimbor—now with added flesh!—has similar abilities to the angry man you play in the main game, but can use The Ring to slow time so he can hit enemies faster. In the books, The Ring is a physical embodiment of the corrupting influence of power, in Shadow of Mordor it gives you bullet time powers. Videogaaaames!
The DLC's out now, and there's a video to prove it.