Upon defeating Duncan the Black, the Hierophant's Chest will spawn which contains three items(or four in HM), two of which are a Deldrimor Armor Remnant and an Onyx Gemstone. Almost all remaining foes and enemy spirits will vanish, and any dead party members will be resurrected. When the last party member opens the chest, a 3-minute countdown to end the instance will begin, after which time the party will respawn just outside Slavers' Exile.
Duncan's monster skill states that 100% of the damage dealt to Duncan is dealt to the source of the damage. This damage can be reduced by skills such as Protective Spirit.
Duncan has not been seen casting his monster skill, suggesting that it is always active.
To avoid the constant attacks from his spirits, there is a corner that works as a shield on the lower floor, from there you can attack Duncan The Black
Bug! Duncan the Black's monster skill states that it inflicts 100% of the damage inflicted upon Duncan on the source of the damage. However, it has actually been observed to redirect 75% to 200% of the damage dealt to him.
Hints for defeating Duncan the Black
Now that Duncan's Defense deals back damage even from such skills as Pain Inverter, Backfire, Mistrust, and Empathy, it is no longer advisable to attack him in this manner. (In the past, these skills were not affected by Duncan's Defense, and were therefore very effective.)
A player with Swap or Gaze of Fury can move his spirits to a much more manageable position or destroy them as necessary. Spirits of Recuperation and Disenchantment should be considered primary targets, but as long as one is swapping spirits, one may as well swap all of them.
Consider using Consume Soul to destroy these spirits instantly.
Players more commonly use Swap. If two party members bring swap, the process is very quick and you can remove all the spirits to a safe spot where they can affect neither Duncan nor you.
Using Recall also helps when swapping the spirits near Duncan; this way you avoid being spiked by Duncan's Channeling Magic.
Many more pop up touchers are added in Hard Mode and can be deadly.
A Spirit-Spammer can easily kill Duncan all alone. Swap all of his spirits away. Then place your spirits upstairs inside attack-range, but outside of Duncan´s aggro-range and make them attack Duncan. Duncan will take damage, but will not move or cast any spells. Heal summoned spirits with Summon Spirits to lower the effect of Duncan's Defense and re-summon spirits as fast as possible. Killing duncan takes about 5-10 minutes with this method. It is recommended not to use Painful Bond in this setting as this damage is thrown back on the caster and casting the spell will aggro Duncan.