Prerequisites: Completed the Path of Dawn quest.
Quest giver: Any Town Guard in Cheydinhal.
Reward: Honorable Knight of the Thorn (only if Farwill survives), the Knights of the Thorn amulet, and the choice of Thornblade or Staff of Indarys from Count Indarys.
The Daedric invasion has spread far and wide across Cyrodiil. Gates have opened at virtually every major town. Farwil and his Knights of the Thorn have entered the gate inside Cheydinhal. His father Count Indarys fears for his son's safety, but must prepare for battle. Amminus Gregori and his men patrol the gate itself. Nothing has come out since the Count's son went inside the gate.
If you want to get the full rewards of this mission, consider bringing healing spells that are Target. You can obtain these from a restoration spell merchant or by making one yourself at an Altar of Spellmaking.
This is a basic clear-the-Oblivion-gate mission. Clear the Daedra all along your way until you meet up with the Farwil and Bremman. This is where it gets tricky. There are a few dremora guarding the bridge to the tower. Do not knock them over the ledge, else Farwil and Bremman decide jumping into the lava is a good thing. Once inside the tower, keep Farwil alive by healing him with Target other healing spells (Farwil also heals himself if you wait 1 hour). If you accidentally hit either of the two, and they begin to attack you, use block and talk to them in order to yield (sometimes Bremman rejects your yield even though he 100% likes you; do your best to avoid hitting them or if you can't help it, use a charm spell on them then try to yield again). They seem to get in the way a lot since they are both melee-based characters. They are also prone to dying so you may want to be their meatshield.
As usual, at the top of the tower, grab the Sigil Stone. After you get back, talk to Farwil. He will give you an amulet, Knights of the Thorn and grant you as an honorary Knight of the Thorn. Additionally, speaking to the Count will allow you a choice between two items: the Staff of Indarys or the Thornblade. Completion of this quest will also grant you partial completion of the Allies for Bruma quest.
If you fail to keep Farwil alive, then you can take the amulet from his body as well as his ring, the Count will then reward you with only the ring.
Alternatively you can avoid talking to Farwil inside the Oblivion gate and head straight towards the sigil stone. Especially when your character is at a high level, it's rather impossible to keep Farwil from dying inside the tower. You also can't tell him not to follow you. Ignoring Farwil means the gates to the tower will not open, causing you to detour through some hot lava, so cast waterwalking on yourself and have some healing potions available. Don't let Farwil see you, or he'll follow you endlessly. Or instead, let him talk to you, and then cancel the conversation once the speaking options appear.
There is a glitch where Farwil gets out of Oblivion on his own without you doing the quest and you'll be rewarded with the weapon by the Count. Afterwards the Count will refuse to talk to you because you "killed his son". Farwil will be grateful while the Breton with him will ask "what is wrong with" you for killing Farwil, right in front of Farwil. The quest is still complete.