Enthir

Enthir is a Bosmer sorcerer and merchant found at the College of Winterhold, and has connections to the Thieves Guild in Skyrim.

Background
Enthir presents himself as a rather shady figure: he calls himself the "guy that can do things." He is often referred to in quests around the College of Winterhold. He can usually be found either in the Hall of Attainment on the second floor toward the northwest in his room or at The Frozen Hearth in Winterhold.

He was close friends with Gallus Desidenius, the former guildmaster of the Thieves Guild. He describes their first meeting as an amusing anecdote, as Gallus broke into his laboratory and, rather than flee from Enthir or fight him, made an interesting comment on his notes. It ended up with them having a long discussion and the two of them becoming friends.

Arniel's Endeavor
Enthir is involved with two parts of this questline: Firstly he sends the Dragonborn to recover the Staff of Tandil in exchange for the Warped Soul Gem ordered by Arniel Gane. Later, after a package goes missing (the unique dagger Keening), a delivery that Enthir insists was organised by Arniel, he directs the Dragonborn to the location where the courier was last seen.

Onmund's Request
Onmund requests that the Dragonborn ask Enthir to return his Amulet, and should they be unable to punch him to do so, Enthir will offer to return the item if the Dragonborn recovers a staff (the Grand Staff of Charming) from a radiant location. Upon the successful recovery of the staff, Enthir will return the amulet.

Hard Answers
The Dragonborn is sent by Karliah to speak with Enthir in order to translate Gallus's journal. Enthir identifies that the text within it was written in the Falmer language, but as he is unable to translate it, he sends the Dragonborn to Calcelmo, self-defined expert on the Dwemer and court-mage of Markarth to have it deciphered. After the Dragonborn collects a rubbing of a falmer deciphering tool located within Calcelmo's Tower, Enthir successfully translates the journal, offering his services as a fence in return.

Services
Enthir sells Black Soul Gems for around 800-1,000. He sells other mage equipment, such as spell tomes, staves and robes, at much higher prices than other merchants. He also sells Daedra Hearts, which are useful for Alchemy and creating Daedric Armor and Weapons. He restocks his items every two days.

After Hard Answers has been completed, he will sell his wares at regular prices. He will only act as a fence when outside the College's buildings or in the town of Winterhold. Be warned: if the Dragonborn attacks him, the Thieves Guild will require a compensation.

Trivia

 * If one compares the Falmer's culture to Enthir's "people" he says he is an Altmer, despite being a Bosmer.
 * There is a dialogue option for Enthir which says "I understand you're an elf who knows how to get things," which may be a reference to The Shawshank Redemption. This goes along his quote "I've been known to handle items of questionable interest from time to time and I'll see what I can do." when he introduces himself as a fence in Hard Answers.
 * Enthir has a misspelled subtitle reading "I undertstand..." in the quest Hard Answers.
 * After starting Onmund's Request, when the Dragonborn needs to talk to Enthir, and he is not found in his room or roaming the college, he can sometimes be found outside the college on a ledge.
 * Unlike other "reputable" mage vendors that only buy magically related materials, Enthir will buy any sellable item, much like a trader or general store. He also charges higher prices for his wares than other vendors, and offers lower prices for items the Dragonborn sells. He charges regular prices after Hard Answers.
 * Enthir is one of the only consistent sources of Daedra Hearts in Skyrim. He will always have two when his inventory resets.
 * Enthir will sell a single Black Soul Gem; however, it will not always be empty and can cost up to 3,000 gold if filled.
 * He is the only Bosmer Fence in Skyrim.
 * Enthir is not a merchant if the player character is unaffiliated with the Thieves Guild.

Bugs
The bad side is that he will start walking back to the inn, good side is that whenever he is needed, one can repeat the process.
 * If the Falkreath Watchtower was cleared for the quest Arniel's Endeavor, and the quest Onmund's Request hasn't been finished yet, the quests cannot be finished since Enthir doesn't talk about anything else anymore. Not even to barter.
 * Possible fix to complete all three quests "Arniel's Endeavor, Onmund's Request, As well as Hard Answers (Thieves Guild).
 * If one has never bartered with him prior to becoming Arch-Mage then he may not barter with them after they becomes Arch-Mage, unless the quest Hard Answers has been completed.
 * Enthir may not talk to the Dragonborn if the quest The Staff of Magnus is active.
 * Enthir may not leave the inn even if the Thieves Guild quest Hard Answers has been completed, and one has not become the Arch-Mage yet.
 * Fix:
 * Go back to the Dwemer Museum in Markarth once Blindsighted has been completed. This should trigger the Hard Answers once again.
 * Finish the quest.
 * Karliah will give the player another Nightingale Blade and walk out. This will NOT require repeating The Pursuit or the ones following it.
 * Once completed, Enthir will return to the College of Winterhold.
 * Wait for him to go back to his room in the Hall of Attainment and he will sell Daedra Hearts once again.
 * Rest or wait a day or so for him to restock.
 * PC users can try this workaround in order to buy the Daedra Hearts:
 * Go to the second floor of Hall of Attainment, where Enthir is supposed to be.
 * Open the console with ~
 * Type prid 0001C1AA (press enter).
 * Type moveto player (press enter).
 * Close console with ~ again, and Enthir should appear.
 * Enthir will complain about some stuff, keep following him and press E to talk to him, he will finally open the dialogue so one can buy Daedra Hearts from him.