Avian is probably the hardest boss you will face in Rookgaard. He is the boss of the fire spirits.
Avian can combo hard up to 500HP so prepare for a hard battle. He shoots some waves in front, some target missiles and his melee attack can be deadly so stay ranged always, heal your self when possible to stay full HP most of the time and just run in circles around the room. Distance ring is recommended
The room is big enough to run around and hit him with distance, but be aware that some tiles will make you walk really slow
He have so much HP, so expect a long fight (might take up to 10 minutes or more) so bring enough supplies of pikes and / or potions.
Don't forget to take the Point of Interest inside the room once you finish the battle.
Avian loot may vary, but some of the most powerful items in Rook are dropped by him: Crusader Helmet, Magma Boots, Fire Sword, Fiery Bow (with higher ilvl than the one from quest), Boots of Haste, among many others.