The Master of Fenris Isle quest chain

The Master of Fenris Isle quest chain is a relatively short chain of quests explaining the presence of undead gnolls in Silverpine Forest.

Prequel: The Dead Fields
The banshee Nightlash has established a base in The Dead Fields. When small expeditions have been sent in the past, some of the Rot Hide Gnolls have been slain, certainly. But each assault has been repulsed by Nightlash's magic, and the gnolls have been reinforced every time.
 * The Dead Fields (quest)

One of the things that High Executor Hadrec wants you to do is to go to the Dead Fields and dispatch Nightlash, returning with what dust remains of her corporeal shell.

The Master of Fenris Isle
High Executor Hadrec tells you that the Scourge have been gathering at the Decrepit Ferry, and he wants you to find out.
 * The Decrepit Ferry (quest)

You journey there, and discover a boat, heaped with corpses! It is clearly evident that wherever these corpses came from, they are headed for Fenris Isle.

A bit of glitter in the boat attracts your attention. On the hand of the recently deceased corpse of a woman, you find a gold ring, with the inscription "For Deliah".
 * Rot Hide Clues

When you bring the ring to Hadrec, he tells you that the only nearby source for such a body would be the mass graves of Tirisfal. The Rot Hide gnolls must be taking these bodies to Fenris Isle.

See also: Deliah's Ring

It is evident that something on Fenris Isle needs to be taken out. But just what that is, and who is behind it, is still a mystery. More needs to be discovered before an assault is made. Hadrec directs you to gather ichor from the Rot Hides on Fenris Isle and bring it to Apothecary Renferrel.
 * Rot Hide Ichor (quest)

It takes a bit (you manage to destroy many of the rot hides too thoroughly to collect ichor from them), but you collect the amount Renferrel needs and bring it to him.

Apothecary Renferrel reduces and analyzes the ichor as best he can, and does discover some of its properties. It is not entirely alchemical, though, and he asks you to take a sample to Bethor Iceshard in Undercity.
 * Rot Hide Origins

With his knowledge of magic, Bethor is able to divine more about the spells involved in the Ichor... and who created them.
 * Thule Ravenclaw (quest)

It turns out that the ichor was created by Thule Ravenclaw, one of Bethor's colleagues who sided with the Scourge when the Lich King rose to power. On the plus side, Bethor knows a lot about the magic that Thule uses, and gives you a scroll that Apothecary Renferrel can use to craft a potion that should counter Thule's more powerful spells.

You return, then, to the Sepulcher, where Renferrel concocts the potion.

Renferrel refers you back to High Executor Hadrec, who has access to the Sepulcher's arsenal. Hadrec concurs with Renferrel that Thule must be dealt with, and offers you some items to improve your armor and weaponry.
 * Report to Hadrec

Armed and ready, you gather friends about you and set off. You have to battle quite a number of Rot Hide gnolls as you work your way through the castle, but finally you come face-to-face with Thule Ravenclaw.
 * Assault on Fenris Isle

Victorious in battle, you retrieve his head as proof that you have slain him.

Returning to High Executor Hadrec, you are praised, and told that fallout of your battle was felt as far away as the Sepulcher.

Rewards summary

 * +825 Undercity reputation
 * Medium Armor Kit
 * 5 of either Coarse Sharpening Stone or Coarse Weightstone
 * Your choice of:
 * High Robe of the Adjudicator
 * Talonstrike (sword)