Dagon Shrine

The four volumes of the Commentaries have led the Hero to discover the Mythic Dawn's shrine to be found in the deep caverns of Lake Arrius.

Walkthrough
It is determined that the location of the Dagon Shrine is in Lake Arrius Caverns. The Hero has two options to go about completing this quest.
 * 1) Infiltrate the Mythic Dawn by pretending to join them.
 * 2) Slaughter all of the cultists.

Infiltrating the shrine
Note: After the quest updates with the message "I have arrived at Lake Arrius Caverns. The secret Mythic Dawn shrine must be somewhere inside" ''if one exits the cave at any time and then re-enters, the player won't be able to get back out. Be sure to obtain the Mysterium Xarxes before exiting the cave or else the main quest will be incompletable!)''

Upon entering the cave, the Hero will be greeted by a doorman who tells them to speak to Harrow. Harrow will require them to give him all of their items and gold. Harrow will lead them into a dark sacrificial chamber where after a short speech from Mankar Camoran they will be invited to join the Mythic Dawn.

They will be prompted to kill the prisoner Jeelius and if this is done they will become an acolyte of the Mythic Dawn. Some of the cultists that were attending the ceremony will leave, making it reasonably easier to escape.

If deciding to release Jeelius, the cultists will attack him and the Hero. If Jeelius makes it out of the cave alive, he will reward them later increasing the Alchemy, Restoration and Mysticism skills by 1 when visiting him in the Temple of the One in the Imperial City. Be warned though, as it can be difficult to keep up with him and protect him since he runs very fast.

Purging the caverns of the Mythic Dawn
Follow the marker in through the cave to the large cavern and kill everyone that attacks. Watch Mankar Camoran do his speech and then disappear before attacking any Mythic Dawn member, as everything picked up in the cave will be considered "stolen". Sneak up behind if possible and dispose of all the Mythic Dawn members and head up on the altar platform where the book is on a pedestal.

Taking the Mysterium Xarxes
Regardless of which method chosen, Mankar Camoran will escape to his Paradise. Picking up the book will open a sealed gate to the north and the escape route. Jeelius has to be saved before picking up the book, because otherwise he'll be killed by the collapsing statue of Mehrunes Dagon. Once the Hero has taken the book, escape from the caverns, and return to Martin in Cloud Ruler Temple.

Trivia

 * Before heading in and getting all items taken away, drop any armor, weapons or potions you want on the ground. Then accept everything to be taken away, run back, and grab the gear dropped.
 * The Blade of Woe, obtained in the Dark Brotherhood questline, will not be taken by Harrow. Players experienced in the use of daggers may want to carry it on the temple.
 * Another option is to sneak past Harrow without talking to him. The cult members inside will not see them as an enemy even if not wearing the mythic garb. To do this, walk just around the corner until one sees Harrow, then walk backwards out of the hallway. When Harrow gets to the end, he will see the gatekeeper and turn around. As soon as he turns around, just run past him and through the door.
 * In the treasure room, down the ramp there is a random ring drop in one of the crates. The Storeroom key found on one of the members can be used to enter. Potentially, it is possible to get Ring of Perfection, which grants a +10 bonus to all stats. Also it may be possible to receive a Base Amulet of Absorption (Spell absorption 14pts).
 * If you give all of your items to Harrow and then decide to kill the Mythic Dawn in the shrine room, your items will all be on Harrow's corpse after he's killed
 * After this quest, in all cities there will be Mythic Dawn assassins attacking the Hero (they will be easy to handle, as some guards and people will help dispatch them). They are named people so they normally have a house that can be looted after killing them.

Dagon-Schrein Santuario di dagon Святилище Дагона (Квест)