Weak to: Bludgeoning damage
Deals: Bludgeoning/Piercing damage
Found at: Elven Grove
Drops: Elven Bow, Rope
Strategy: 'Marcus sent you' is the safe option.
Created to be a challenge for veteran players to test their skills and their luck, Dir'Thelien with a strong Elven Bow is unquestionably the strongest boss monster of them all. Dir'Thelien will kill anyone who fights it long enough. Players are recommended to stay far away from this boss, killing this boss is not required to progress in the game.
Despite his weakness to bludgeoning type damage, it is recommended to use powerful weapons with significant block chance and damage reduction - usually this will be Slashing type weapons - as survival is your first priority when considering fighting Dir'Thelien. It's also required to have multiple characters and sets of equipment. Possible gear choices include the Sword of the Black King, Emerald Sword or Knight's Longsword alongside a high-end Protector gear, or a top tier set of Paladin Armor. At the cost of higher risk, very strong bludgeoning and piercing weapons can also be used.
It is absolutely necessary to have mastered the basics of Initium to succeed against this boss. Barring the use of an army of fresh characters armed with Shurikens, it is expensive to even attempt to kill Dir'Thelien considering he lands strong critical hits frequently because of his high stats (13 Strength and Dexterity, 12 Intelligence).
Reference Vocandin's Boss Hunting Guide for a comprehensive list of all bosses.