This item drops from Terokk. To summon him is an arduous task.
First, head to the Skettis area of Terokkar Forest where folks farmed Sha'tari Skyguard reputation for their nether ray mounts back in the day.
To start the chain to summon him, head over to Severin to do World of Shadows (or the repeatable alternative More Shadow Dust if you've done it before) to get Elixir of Shadows. After you have your elixir, pick up Secrets of the Talonpriests from Sky Commander Adaris who occupies the same tent as Severin. You will need to be under the effect of the elixir to complete this quest. While under the effect, save any Time-Lost Scrolls that drop - you will need them later.
When you kill Talonpriest Ishaal for the quest, he should drop Ishaal's Almanac. When you turn it in, Adaris will give you An Ally in Lower City, which leads to Countdown to Doom. After turning that in, the Arakoa in the cage by Adaris and Severin will offer you Hazzik's Bargain. Complete that, then A Shabby Disguise and then finally you will get Adversarial Blood.
You will also need x 40 Time-Lost Scrolls to complete Adversarial Blood. These drop from mobs all over the Skettis area while under the effect of Elixir of Shadows, meaning you may need to farm more Shadow Dust. As you kill each mob required for the quest, save the body parts they drop ( Gezzarak's Claws, Vakkiz's Scale, Karrog's Spine, Akkarai's Talons), as you can use these to get a second Time-Lost Offering to summon Terrok. Also note that the bone piles take 15 minutes to respawn, so you will need to move to different ones for each miniboss.
When you're all finished and have your Time-Lost Offering, take it to the large rune and bone pile on Terrok's Rest, the island in the middle of the lake to summon the boss. His bone pile also has a 15 minute cooldown, so if you intend to fight him twice, you'll need to rest between.
After you've done all this before, you only need to collect the x 40 Time-Lost Scroll (which does require an Elixir of Shadows) so you can get Gezzarak's Claws, Vakkiz's Scale, Karrog's Spine, and Akkarai's Talons from the bonepile mini-bosses to turn in for Tokens of the Descendants to get another Time-Lost Offering.
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.