Mirabelle and Tolfdir hove retreated back, unable to stop the tide of harmful magic which Ancano has released from the Eye of Magnus from sweeping across the College of Winterhold. The Staff of Magnus from the Labyrinthian ruins is needed as quickly as possible.
Make your way to the spot indicated on your map where the vast and sprawling remains of this ancient Nordic city are located. Enter the surface courtyards from the mountains above, or the forest below, using the stairs and the dried aqueducts.
As you approach the entrance to the ruins, you will see ghostly apparitions of the deceased arch-mage Savos Aren along with five other apprentices from the College of Winterhold who once accompanied him on a mission to these ruins.
The apparitions will not acknowledge you, and will instead live out their final moments. Throughout the traversal of the dungeon, there will be fewer and fewer apprentices. The arch-mage will demand that they all press on, until finally he is the only one left. He isn’t powerful enough to fight and win against the Dragon Priest (as is noted by the omniscient dialog the Dragon Priest will give throughout the dungeon). At several points throughout the dungeon, the Dragon Priest will speak to you and when he does, he will drain your magicka, so magic reliant characters might want to make sure they’re carrying a good supply of magika replenishment potions.
Once the first sequence of apparitions disappear, move to the Ceremonial Door and activate it. You automatically remove the Tore of Labyrinthian and slot it into the door. The entrance opens for you.
A Frost Skeleton Dragon will rise up from the earth to attack you in the first room of Labyrinthian, along with about ten skeletons. The skeletons are very weak, especially against fire magic, so it’s easy to take them out first before concentrating on the dragon. The best way to defeat them all is to conjure Flame Atronachs through the portcullis without ever opening it. This way you can have your minions kill all of the skeletons without sustaining any damage yourself and be fully prepared to face the dragon. The dragon is also highly susceptible to fire magic, and the Fire Breath dragon shout, if you have it, will knock out the majority of his health.
In the room inhabited by the Draugr and Frost Spirit, blasting the door from which the Frost Spirit attacks will open it for you. This door requires a fire spell to open, but fortunately one is provided for you on the nearby altar. Later when you confront the Fire Spirit, there will be a nearby tome with a frost spell which you can use that to open his door.
There is a trapped hallway filled with triggered towers casting Frost Bite, Fireball, and Flames spells; along with maybe a few Frost/Fire runes. On the higher difficulty levels, this can be quite a challenge for a caster.
The best way to get past this bit is by taking the soul gems out of the first two towers, dodging the Fireballs being cast by the end tower. and then running to the hallway on the right. Save the game often in this hallway if you are having difficulties. You can also disable the towers by knocking the soul gems off their pedestals; Ice Spikes are one of the spells that are good for that, and arrows might also work.
There are also Wisps that you’ll encounter here. Be sure to concentrate on killing the Wisp Mother, as all the others emanate from her.
Eventually you’ll come to a leveled Draugr sitting on a throne opposite to a Word Wall. It can use Disarming Shout so handle this with care. Kill it as quickly as possible, ideally from a distance. Once it’s defeated, read the engraving on the wall and learn one of the words for the Slow Time Dragon Shout.
Eventually you’ll witness an apparition of the dead Arch-mage and two remaining apprentices. When you see this you’ll know you’re about to face the source of the evil in this place: Morokei the Dragon Priest.
Enter through the doors to the ceremonial chamber, where Morokei has been sealed in an impenetrable magical barrier by two Wizard Thralls. There is a beam supporting Morokei’s barrier, and the barrier gives the Dragon Priest invulnerability to your attacks. Interrupt the thralls from their ritual by striking or killing them to break the beam. Once his barrier goes down, Morokei becomes vulnerable and attacks you. He’s now yours for the taking.
When you defeat hit, make sure you take the Staff of Magnus from his corpse. You can also pry off his mask, which is important for completing the side quest Masks of the Dragon Priests. There are also three chests in the final room, including one underwater which is protected by a single Slaughterfish; there are also Slaughterfish Eggs to be harvested on the stairs of the pool.
Exit from the ceremonial chamber via the eastern doors. You will see one final vision of Savas Aren on the way. Continue up the stairs and open the portcullis using the wall lever.
As you step through the portcullis, a Thalmor agent named Estormo appears and informs you that Ancona was correct; you are a threat to them. He has instructions to get the Staff of Magnus from you and to kill you. So begins another battle. Deal with this fool and navigate your way out of another crypt, up the southern stairs and back outside. Head back to the College.
Back at the College, the surviving members have pulled back to the bridge from Winterhold. When you ask Tolfdir where Mirabelle is, he tells you she didn’t make it; she died to save the others. Tolfdir is now in charge, but none of the College members can come close to countering the magic now at Ancona’s disposal. With the Staff of Magnus in your hands, however, there may yet be a way to stop him and save the College and the city of Winterhold.