From the east road, the road turns north and takes to the Cemetery. If you have completed the Slayer Quest, you will appear just west of the entrance to the Cemetery. After entering the Cemetery, just follow the dirt trail, or go northeast to come to a building.
Mythran had directed you to seek the Necromancer, and you see a mage outside the building. Talk to him. He is Vividos, and he will tell you about the history of the Necromancers. He will tell you that Lothian is the current Necromancer, and he will also tell you that Mordea stole a dagger. Offer to retrieve the dagger, and you now have a new quest - to get the dagger back.
Aramina, the Tempest's servant with the white dress, has access to the dagger. However, she will not talk to you if she is on duty at the palace, so you must meet her at her house (if you have trouble spotting her house, there is a little map on the Ultima Web Archive showing where it is) at Bloodwatch (check times of day if you're not familiar with them). When you talk to her at her house and ask about the dagger, you must tell her she won't get into trouble before she will give you the key.
Now that you have the key, you can get to the dagger. Go to Central Tenebrae and into the palace. If it is still Bloodwatch, Mordea will be asleep, so go into the throne room, and take the key from under the skull cushion. There is a map of Mordea's Bedroom (Internet Archive) from Pentology: Ultima VIII Cluebook that can help you locate it. Now, use that key to open one of the doors to Mordea's bedroom, behind the throne room. If she is asleep, do not step on the carpet but walk around the edge on the west side, or else you will wake Mordea up and you will get a shocking experience. Open the door and the chest with Aramina's key, and take the dagger. You can also help yourself to the Restore to Sight and invisibility scrolls in the chest.
Now that you retrieved the dagger, you obviously need to take it back to Vividos. Go to the Cemetery, and Vividos is inside the building, beside Lothian who is on the altar. Talk to Vividos and give him the dagger, and go get some popcorn before watching the Ceremony of Eternity, in which Vividos uses the dagger to kill Lothian. If the ground shakes infinitely, then there is a technical difficulty which you can surpass by turning off the sound, which you can turn back on when the ceremony is over.
After Lothian's death, you can question Vividos. After asking if he is the Necromancer, you can also learn what a Scion is. Accept to become his apprentice, and he will send you to get some reagents for him.
The first reagent he wants is "sticks from a place where spirits roam", which is in West Tenebrae as you will learn if you ask him. He also wants Executioner's Hood. So you leave the Cemetery to search for the reagents. Where the East Road ends in East Tenebrae, you are stopped by a guard and told some shocking news...