A Venerable Vintage
I have to confess that this is probably my least favorite quest. Not because it's particularly difficult to complete, but because it's a major pain to begin unless you start with the quest-giver and even then it comes down to sheer luck without a walkthrough of some sort. Although you can (and probably will) begin with the quest object, it is a dead-end.
At some point in your dungeon-delving, you'll pick up a couple of bottles of something called "Shadowbanish Wine" while looting one of the 40-odd forts around Cyrodiil. When you pick it up, you'll get a message telling you that someone is probably interested in these bottles. The big question is "who?". Unlike the Nirnroot quest, you don't pick up any conversation topics about it until you talk to the quest-giver and, because of the lack of conversation topics, finding the quest-giver is one of the two hardest parts of the quest.
One of the rumors that you can pick up in Anvil concerns Wilbur at the Count's Arms Inn, who is rumored to have a good knowledge of fine wines. It may have been intended as a pointer to the real quest-giver at some point in the development process, but it's a dead-end now.
But if you persevere in your search, you'll eventually find Nerussa at the Wawnet Inn, just outside of the main gate to the Imperial City. She has a "Wine" topic and will tell you that the Shadowbanish Wine was a special brew for the Imperial Legion that had a Night-Eye effect for soldiers on night guard duty. She'd like six bottles, if you can manage it, and will pay you for them.
Now that you've found the quest-giver, it's time to hustle up the other bottles. This is the other hard part of the quest. Like "The Collector" quest that sends you searching through the various Ayleid ruins scattered across the map to find a double-handful of statues, this quest sends you digging through 40-some-odd forts, looking for the eight that have what you need. Each of the eight forts will have a chest that contains two bottles of Shadowbanish Wine. Since you only need six bottles to complete the quest, it's really just a matter of finding two more forts, rather than all eight. A very small consolation, but better than a poke in the eye with a sharp stick.
The eight forts, in alphabetical order, are:
Fort Aurus. It's just south of where the Corbolo River empties into Niben Bay. It's a Conjurer hang-out and the chest is in the room farthest east on the map.
Fort Carmala. It's south of Chorrol (you probably passed it on your way to Hackdirt for the "Shadow Over Hackdirt" quest). It's also a Vampire lair. The chest is in the far northeast corner of the dungeon with a Vampire Patriarch keeping an eye on it.
Fort Dirich. It's kind of southwest from Weatherleagh (from the "Legacy Lost" portion of the "Separated at Birth" quest in Chorrol). It's an undead lair (in case you need that little something for Meridia) and you'll find the chest in the southernmost room.
Fort Grief. It's on an island in Niben Bay and is the scene for the "Caught in the Hunt" quest in Bravil. The chest is not in the dungeon, though. It's on the top floor of the tower.
Fort Irony. Right next to where you talk to the Grantham Blakeley in the "Forlorn Watchman" quest in Bravil. He appears in Bawnwatch Camp (just south of Bravil) every night at 8:00. Follow him and you'll walk right by it. It's a one-level goblin lair and the chest is in the southernmost room.
Fort Magia. It's a bit south of Wellspring Cave (Mages Guild "A Mage's Staff" quest) on the eastern short of Lake Rumare. It, too, is an undead hangout. The chest is on the bottom level (Garrison Blocks), next to a sarcophagus in the northwest corner of the level. It's partially submerged.
Fort Selnia. It's southeast of Cheydinhal, in the mountains near the border. It's at the east end of the entry passage of the second level (Hall of Legends) and should be unguarded.
Fort Vlastrus. If the name weren't a clue, it's a Vampire hideout. It's just off the Gold Road, a bit east of Skingrad. The chest is on the north end of the dungeon and has a few guardians, including a Vampire.
Once you've managed to find your six bottles, take them to Nerussa to collect your reward (1000 gold). Since there are still ten more bottles out there, you can bring them to her and she'll pay you 100 gold for each, for another 1000 gold. The potion has a 4-hour (game-time) Night-Eye effect, but I'm not sure that it's worth the 2-hour 10-point drain on your Intelligence and Agility. Probably better to just combine some Carrot and Viper's Bugloss leaves and whip up your own.