I was able to get through to Storm Peaks and start the questline.
You meet with Mimiron in The Inventor's Library, which is south of Ulduar. There you find that Mimiron also called upon Grif Wildheart, who was the dwarf that joined us when we originally acquired Titanstrike.
Mimiron then starts to triangulate data using the Thunderspark (the powersource of Titanstrike) to try and found out where Hati is. As he does so, rifts to other locations begins to open and send hostile proto-drakes and vrykul out to attack you.
After this Mimiron is able to determine that Hati's essense has been split and can be found at places where he committed great sacrifice. The first location is the Temple of Storms in Storm Peaks, which is where Hati took the brunt of the explosion when Titanstrike was overloaded.
However, more rifts open up and Silithid start to pour out and attack you and Grif. (I did check these to see if they're tamable. They're not).
After defeating the Silithid you are then sent to Storm Peaks to meet with Thorim and see if that part of Hati's essence can be found.
When you and Grif arrive at the Temple of Storms, you find that Hati has slain many Vrykul and Proto-drakes, and has also bound Thorim in lightning.
When you're ready to start the encounter, you talk to Grif whom calls out the Hati. However, the latter does not recognize Grif nor you and begins to attack.
The fight itself wasn't that difficult. It may have been do to my ilvl or that the fight itself wasn't designed to be particularly difficult. Hati will focus on Grif's bear or on your pet most of the time. He will occasionally send a lightning strike in a line towards you, which hurts if you stand in it. He will also cast Titan's Spark, which is an interruptable spell that deals ~5-10% of your health in damage. He will also focus on you and chase you down for a short time, but a Concussive Shot will slow him and prevent him from catching you.
When you get Hati to ~10% of his health, he will become Unfriendly and unattackable. You are then able to use the Thunderspark to absorb that part of Hati's essence into it.
Once that is done, you are told to travel to Silithus to find the other half of Hati, which is fading fast.
I'll start documenting it.