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This boss is summoned in the Skettis area of  Terokkar Forest where folks farmed Sha'tari Skyguard reputation for their badass 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.

Back when The Burning Crusade was current, this boss was a veritable pain in the you-know-where. Not only was it a grindy process to summon him, but you would also have to gather the materials again if your group wiped (which was painful in itself). Generally, players only bothered with him at all because of the Patch 2.1 quest Terokk's Downfall -- which gives a hefty 1k rep reward for the Sha'tari Skyguard.

Now that a lot of time has passed since those days, players can easily obtain the materials to get a chance at the  Time-Lost Figurine.