You will find Marcellis and Alessia on the bridge to the northeast of Somborn Village around ( -555, 15, -79 ) /waypoint -555.29, 15, -79. Marcellis attacks after a few seconds. He's a Heroic ^^^ but I managed to kite him each time. Bring a group for best results*. Around 20% health he will despawn.
Once they despawn you will be timed on each of the following steps.
Chase Marcellis and Alessia. (Note: You can ask the guards for locations)
The first location is near the end of the river at ( -73, 10, -404 ) /waypoint -73, 10, -404. He will attack again, despawning at around 20% health.
Keep chasing Marcellis and Alessia.
The next spawn point is at the bottom of the stairs on the first platform headed towards Mistmoore Castle. ( 701, 20, -24 ) /waypoint 701, 20, -24
Escort Alessia back to Somborn and Aaron. (Note: You can fly or evac back to Somborne)
Aaron is standing near a tree at ( -425, 10, -27 ) /waypoint -425, 10, -27