In Warlords of Draenor, this item will be added to the new Toy Box feature and no longer take up a bag slot. You will be able to "use" the item straight from the Toy Box interface.
The Toy Box will be account-wide and therefore so will this item.
Kalytha's Haunted Locket may be purchased from Tex Vortacoil without having completed any prior Azshara quests.
The spell changes your character model to that of Spirit of Kalytha. The change is considered a transformation (meaning that you can use abilities, use mounts, et cetera). It stacks with other model-changing abilities, items, and trinkets, though not all. R.I.P. druids and shaman (and demo locks) as usual. :(
Of important note: The buff has a duration of 30 minutes while the item's cool-down is only 15 minutes. This is one of the rare instances in which a toy has a duration greater than its CD. 5K gold well spent!
For information about Kalytha, see the Wowpedia articles for the quests "Memories of the Dead" and "Mystery of the Sarcen Stone".