Magni told you about Destroyer Hamitemon, the leader of the Petrahulks. Get rid of him to help save the Red Lava Cliff trees.
Step 1. Kill Destroyer HamitemonThe Alpha Petrahulk. He roams The Crash Site. (x1).
Step 2. Talk to MagniAn Elim at Red Lava Cliff in Morheim. He became an Agrint but was cured by a bolt of lightning..