The Hoarfen was produced by the union of Fenris Wolf and the sister of the Frost Giantess Siingard. It's existence was kept secret as the monster was used to guard the castle of Siingard, though whispered rumors of its existence leaked out.
When the Hulk, Betty Banner, the Warriors Three, and a few members of the Pantheon pursued Agamemnon to Siingard's castle, Siingard unleashed the Hoarfen on them in an effort to protect Agamemnon. Siingard blew the horn to summon the beast, commanding it to kill all down below, but to kill the green one first. Just then, Hector flew in and snatched Agamemnon, but the thunderous approach of the Hoarfen made Hector drop Agamemnon into the Hulk's hand below. Unwilling to listen to Siingard's commands to stop, the Hoarfen grabbed the Hulk and Agamemnon in his jaws, killing them both.
The Hoarfen remained relatively unphased by the combined attacks of the other invaders, but when the Hulk and Agamemnon were restored to life by Hela, they reappeared in the surprised monster's mouth. The Hulk forced Hoarfen's mouth to open too widely, fracturing his mandible, and the Hoarfen fled the scene.