Erandur

Erandur is a Dunmer who can be found at the Windpeak Inn in Dawnstar. Early in his life, he was inducted into a cult dedicated to the Daedric Prince, Vaermina. After he grew up, he left the order, changing his name from Brother Casimir to Erandur, and dedicating his life to Mara.

During the Civil War, the Dragonborn may have assisted him with eradicating Vaermina's cult, after several citizens of Dawnstar fell victim to gruesome nightmares. After, he may have joined the Dragonborn as a follower, potentially becoming recruited into The Blades.

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.

Waking Nightmare
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 the fire spells Flames and Firebolt, and switches off to a leveled mace when in melee range. He also uses a Ward occasionally when in close combat, often in conjunction with the Flames spell. He sometimes uses a bow to do battle from a distance, mostly if his spells can't reach the enemy.
 * If given a one-handed weapon, Erandur will often dual-wield it along with his default mace in combat once he runs out of magicka, though this stops altogether if he is given magicka-restoring potions and jewelry that assists with casting spells.
 * 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.
 * Erandur is loyal to the player character once he is recruited. For instance, if the player gets into a fight with a Hold's guards, Erandur will fight the guards alongside the player.

General quotes

 * "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."
 * "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."
 * "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."
 * "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."
 * "Finding Mara was the greatest moment of my life. I'll never forget the warmth that spread through my heart when we embraced."

Trivia

 * While Erandur prefers to use Destruction spells and one-handed weapons, these are his lowest combat skills. The  console command reveals that Conjuration is his highest skill (level 63).
 * Choosing to banish the Skull of Corruption and let Erandur perform the ritual makes the Oblivion Walker achievement impossible to obtain, because as there are 16 Daedric artifacts, and the Skeleton Key does not count towards 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.
 * Interestingly, Erandur's skin color changes depending on what he is wearing: it being a dusty blue shade if he is wearing his default robes or any hooded robes, and a grey with light purple tones if he wears light or heavy armor. The reason for this is unknown.
 * During the cleansing of the Nightcaller Temple, Erandur has a few lines of unique combat dialogue, such as: "For Lady Mara!" However, if he is saved at the end of the quest, he will no longer use this combat dialogue, and will instead mostly use standard Dunmer combat dialogue.
 * Checking his inventory after he is spared during Waking Nightmare will reveal that, as well as his default gear, he is also carrying 8 torches. These aren't marked as "his," and so they can be taken freely.
 * Although Erandur prefers to cast spells, his magicka pool is actually very small, and it does not increase as he levels up.
 * As Erandur levels up, his default weapon (a mace) will change every few levels, alternating between heavy armor and light armor maces (e.g. a dwarven mace to an elven mace). At levels 43+, his mace changes to an ebony mace, and this mace will not change even when he is at max level (50).
 * When he casts the Flames spell on the totem of Vaermina in Nightcaller Temple, he uses a unique animation; his palms face up when casting the spell. However, if he is saved, he does not use this different animation again when dual-casting Flames.

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 make 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.