You recieve this locket after finding all the pages in the Soul Cairn fo Saint Jiub. What is its effects?

50 stamina and 50 carry weight. Nothing special. The quest is just an easter egg for morrowind fans so they dint complain about getting ripped off with this dlc.

oh yeah that sounds meh, does it have a unique look?

Yeah like these guys said 50 stamina and carry capacity, however it doesnt have a unique look, its just a gold amethyst necklace, but if your planning on hoarding most of the stuff you find in your Dawnguard travel's youll need it... saved me more than once!