Erandur

Erandur is a male Dunmer, found at the Windpeak Inn in Dawnstar.

Early life
Erandur was recruited into the cult of Vaermina as a young elf and wasn't allowed to socialize with people outside of the cult, which led to him having what he calls 'no childhood to speak of'. At some point, Erandur was a member of the Bards College before his affiliation with Vaermina was discovered.

Cult of Vaermina
He grew up within the cult and lived in Nightcaller Temple until several Orcish invaders, tormented by nightmares left behind by Vaermina, attacked the temple. Realizing that they couldn't hope to defeat the Orcs and desperate to keep the Skull of Corruption safe, the priests decided to release the miasma, a mist of sorts that puts people into a deep sleep. Erandur (then named Brother Casimir) was the one to release it, but before the miasma could take him, he fled, leaving his fellow priests to die. It was an act that haunts him to this day.

Priesthood of Mara
He spent a few years wandering Skyrim until he was taken in by a priest of Mara. He decided to become a priest of Mara himself, although he did have trouble adjusting from the dark and brutal ways of Vaermina to the kind and loving ways of Mara. His fellow priests patiently helped him through the process, and now he has gone back to Dawnstar, where he has to deal with Vaermina's influence on the people.

Interactions
Speaking with the Jarl of Dawnstar, the Dragonborn learns of a fatal condition pervading the citizens of Dawnstar. Several fall asleep indefinitely while others complain of dreadful nightmares. Finding Erandur in the Windpeak Inn, the Dragonborn agrees to follow him to Nightcaller Temple and end the streak of nightmares. There, it is revealed that the Dragonborn must enter the memories of Erandur, using Vaermina's Torpor, an alchemical draught, which reveals Erandur as a former Vaermina cultist. Succeeding, Erandur and the Dragonborn proceed toward the altar of the temple, hoping to destroy the Skull of Corruption and end the nightmares occurring in Dawnstar. The two are attacked by Veren and Thorek, priests of Vaermina, who accuse Erandur of fleeing from the battle, and not falling into a paralytic sleep. Defeating them, the Dragonborn has the option to allow Erandur to perform a ritual and destroy the Skull of Corruption or take it for themselves. Before making the decision, Vaermina calls out to the Dragonborn, promising great power if he or she chooses to slay Erandur. Choosing to obey rewards them with the Skull of Corruption, while refusing opens Erandur as a follower.

Follower

 * Erandur dual-wields Flames spells, switching off to a leveled mace when in melee range. He occasionally uses a bow to do battle from a distance.
 * Erandur wears simple robes and will not equip enchanted mage robes, even if the enchantments exceed the value of his default equipment. He will, however, equip Light Armor or Heavy Armor given to him, as well as any weaponry superior to his default leveled mace. Having Erandur equip additional armor requires that a helmet or circlet be given to him first. The Console command  allows him to equip any type of item given to him. However, he reverts to his default equipment if he is relieved of his services.
 * Erandur can be inducted into the Blades once the Dragonborn escorts Delphine and Esbern to Sky Haven Temple. After, he resides in Sky Haven Temple and his default armor switches to Blades armor.

Quotes

 * "Markarth has quite the reputation due to the Silver-Blood family. I wouldn't advise crossing them."
 * "I was once a student at the Bard's College, until my affiliation with Vaermina was discovered. I haven't been back to Solitude since."
 * "These poor souls in eternal servitude..." (about the Draugr in Nord ruins)
 * "I have seen the worst of dark magic up close as a priest of Vaermina." (while among dark mages in a Dungeon)
 * "You dare fight a Dunmer?!" and "Son of a..." while fighting.
 * "A good friend of mine was infected by a vampire long ago. He killed himself rather than change into one of those… things."
 * "It's wonderful to be able to sleep soundly, don't you think?"
 * "I've been enjoying this journey with you. Not for the wealth we've accumulated, but just being able to carry Mara's will to every corner of Skyrim."
 * "Trust in Mara… She'll protect us in this dark place."
 * "It was difficult to come to the ways of Mara at first, but the fellow priests were kind and quite patient with me."
 * "I always felt a great deal of guilt for running from Nightcaller temple and leaving my friends to die. I'll never forgive myself for that."
 * "Bandits know nothing of pity or mercy. No need to show them any, either."
 * "I'll see you burn!!"
 * "I've done a lot of… questionable things in my life. Serving Vaermina was a horrible mistake, and I hope Mara will forgive me one day."
 * "Impressive. Damn impressive."
 * "I was recruited as an acolyte of Vaermina as a young elf. I had no real childhood to speak of… we weren't permitted to socialize."
 * "I've never admitted this before, but I've killed more than I care to admit. I'll just leave it at that."
 * "I grew up in the Pale. This land was my home for most of my life… and for a Dunmer, that's quite a statement, I assure you."
 * "When I was a young acolyte of Vaermina, I paid my dues in a shrine located within the catacombs of a fort in Cyrodiil."
 * "I served as a healer in a fort not unlike this one not long ago."
 * "Finding Mara was the greatest moment of my life. I'll never forget the warmth that spread through my heart when we embraced."
 * "Why anyone would choose to reside down here is beyond me."
 * "These caves… They harbor ill omens, I can sense it."
 * "What a marvel of engineering. The ancient Nords were ingenious builders."
 * "Watch yourself, these Nordic ruins are notorious for their traps."
 * "Ugh, this place… horrible."
 * "These ruins are old, maybe a thousand years or more."
 * "The Forsworn haven't been kind to Karthwasten. I've heard rumors that they've attacked this place without mercy."
 * "I've seen my share of black magic, but that's all in the past now."
 * "Let's try and stay together, the sorcerers in these places tend to… well, harvest bodies for parts."
 * "Did you hear that? A metal on stone sound… hmmm, perhaps it was nothing."
 * "This place could be massive… very easy to get lost. We have to be careful not to lose our way."
 * "I can't imagine what happened to the Dwarves. How could a race with the capability to build a place like this simply vanish?"
 * "I've never been inside a Dwarven ruin. They are simply astounding.
 * I've always wanted to make a pilgrimage here to the Temple of Mara. Thank you for bringing me here... you've made me quite happy. (When he is taken to the Temple of Mara in Riften)

Quests

 * Waking Nightmare

Trivia

 * While Erandur prefers to use Destruction spells and One-Handed weapons, these are his lowest combat skills. The "getavinfo" console command reveals that Conjuration is his highest skill (level 63).
 * Choosing to banish the Skull of Corruption and let Erandur perform the ritual does make the Oblivion Walker achievement impossible to obtain, because as there are 16 Daedric artifacts, the Skeleton Key does not count towards the said achievement. It may be possible to get both artifacts from the Hircine quest, enabling the achievement, but it has not been verified that both will count towards the 15 required.
 * Erandur, as well as Malyn Varen and several other Dunmer males are voiced by Keith Szarabajka.

Bugs

 * Sometimes when speaking to the Jarl of Dawnstar, the dialogue option relating to the nightmares haunting the people of Dawnstar isn't available. Speaking with Erandur at Windpeak Inn instead sparks the quest.
 * Upon entering Nightcaller Temple during "Waking Nightmare", Erandur may teleport to the Dreamstride, causing the quest to be impossible to complete.
 * To resolve with console commands enter "prid 0024280", then "moveto player". This teleports Erandur to the player's location.
 * When in the Nightcaller temple he gets stuck on the steps and will not move forward.
 * To fix this, sprint into him to back him stumble backwards, up the stairs. Once at the top he should return to normal.
 * Using Unrelenting Force to throw Erandur to the top of the stairs will also work to unstick him.