When fighting Derwen, he can use a special attack which will release multiple balls of energy in the area around him. These balls of energy will heal Derwen for five Hitpoints every few seconds. They can be destroyed by the player, having 20 hitpoints each. He can also teleblock the player for one minute.
Derwen can only be harmed by the Claws of Guthix spell. As such, a Guthix staff, a staff of balance, or the void knight mace is required to defeat him. Despite being referred to as an ent, killing Derwen will not count towards an ent slayer task from Krystillia.
^Kolodion. Old School RuneScape. "[Derwen] seems to take great interest in the other two creatures. Maybe he is trying to keep a balance between them."