Speak to Grif Wildheart at Krasus' Landing in Dalaran.
I know the power of Titanstrike well. The Unseen Path battled the vrykul warlord Volund as he campaigned to conquer the Broken Isles. We sought to wrest the rifle from him, but he took it to his grave.
If Volund's tomb can at last be opened, the artifact would be a great boon to our cause.
I believe Grif Wildheart is making preparations to depart. You can meet him at Krasus' Landing.