The Arboreal Warden can be found under the archives. Four Valaran soldiers guard the entrance of the cave in which he resides. Upon death, he drops an Agrestal Blade.
The Arboreal Warden is a huge, tree-like boss, as evidenced by his name. The boss is situated at the back of a large cavern and is both immobile and unreachable. When killed, the Agrestal Blade appears in a bunch of vines on the ground.
The Arboreal Warden is not a difficult fight, but requires patience. Melee fighters will need to run right up to him and wait for one of his ground-pound attacks. One of these is a punch which deals heavy damage and will only leave the warden's fist on the ground momentarily. The other is more highly telegraphed and can be identified by the swirling green energy that the warden conjures before bringing down his fist. After this attack, the warden will release the energy and slow the player.
Magic characters do not need to worry about waiting for the warden to attack. Simply lob spells such as fireball or arc up at the boss and dodge his attacks and he will go down relatively quickly.