Establish a portal to the Twisting Nether at the Stair of Destiny in Outland.
Kruul boasts damnation, but his power has not yet fully manifested. His minions have taken his heart to hasten his rebirth in the nether.
We do not have much time.
Go to the place of Kruul's fall at the Stair of Destiny in desolate Outland, where the link to his soul is strongest. There you must pry open the way to his realm.
I must prepare for the battle to come. Good luck, champion.