History
Treants are an ancient race of tree-like creatures who served the god Cenarius, Lord of the Forest. As such, they are often associated with druids, which is evident to players with this class in World of Warcraft. In the distant past, when the Burning Legion invaded Azeroth, their demonic powers influenced and corrupted many of the treants, turning them evil and driving them mad. As a result, Corrupted Treants in World of Warcraft are now hostile non-player characters (NPCs) that will attack the player.
Location
Though there are many Treants located throughout Azeroth, there are only two types that are labeled as corrupted. The Corrupted Treants are located on Bloodmyst Isle, which is the secondary zone after the Draenei starting area. Look for them in the southern central to central areas of the island. The Corrupted Darkwood Treants appear in the higher level Blasted Lands zone, just on the other side of Deadwind Pass. You can find them in the southwestern part of the zone in an area called The Tainted Forest.
Level and Abilities
Since the Corrupted Treant appears in a low-level zone, these creatures range in level from 11-13. While this might seem low, you also will usually be of equivalent level when fighting them, so be careful to not fight more than one at a time unless you are confident you can win. Other than basic melee attacks, they can cast Stone Skin on themselves, which reduces damage done to them while slowing their movement speed. The Corrupted Darkwood Treants are much higher level at 59 and can cast fire spells called Fel Fireball and Fel Immolate.
Quests
For the quest called Victims of Corruption, you must collect 6 Crystallized Bark for the NPC Morae. When you kill a Corrupted Treant, one of the loot items will be a Crystallized Bark. As you kill these creatures, be careful to keep moving. They respawn very quickly after they are killed, and can re-appear and attack you again. Both Horde and Alliance must complete a quest in The Blasted Lands that requires you to kill several different types of hostile creatures. The Corrupted Darkwood Treants are one of them and you must kill 5 of them, along with other creatures, to complete the quest.