To finish quests: 

“Assassinate: Shields Hallows”

The Syndicate has identified Inquisitor Matthias to be a target for elimination. Prove your cunning by tracking them down in Shields Hallows and destroying them.

“Held Below the Surface”

Confront Inquisitor Matthias, now among the Lost outside Brackwater, and bring his Medallion and Soul Candles to Justiciar Farrell in the Brightwood Settlement.

You would need to kill npc Inquisitor Matthias, that looks like this:

He is located exactly as it is shown in the quest, on the map:

But before you jump in to kill Matthias, be very careful, because this NPC located near border of high level location, which causes monsters of lvl 50+ run around this spot, if you encounter wolf with a skull, depecting it danger lvl – RUN AWAY, it can kill you in 1 hit!

To watch all about this npc & quest you can check out this youtube video: