The Murkwater Catacombs is found in the eastern portion of Western Limgrave, by following the water north-east from Agheel Lake.

As you enter the Catacombs, you will immediately find the Site of grace. Head down the stairs and you can examine the object to your left. Then it will tell you this summoning pool is now functional.  By heading down a little longer, you will be welcomed by a couple of Fanged Imps that dwell on the roof of the room, you can choose to attack them before they are animated. These Fanged Imps may also appear as static figures in the corners of the room. You will find 2 Grave Glovewort (1) in this room. Keep moving forward.

If you head straight towards the room with the statue, beware of the square platform on the floor, as this will activate a trap shooting 3 sets of 3 projectiles. If you do activated it, stay close to the walls to avoid them.

There will be more Fanged Imps in the area. Deal with them, and you will find Root Resin on the ground and a lever against the wall. Push the lever. Turn back and head to where the path forks and take the other way down. There are more Fanged Imps waiting for you in the next room, two on the ground and another one hanging on the roof waiting for you to enter.

Once you are done with them, you will reach the set of doors leading down to the fog gate boss area. You will find the Grave Warden Duelist there.

BOSS BATTLE: GRAVE WARDEN

The Grave Warden Duelist is an optional boss and probably one of the easiest to beat. As soon as the battle starts, summon a Spirit and let the spirit companion do the hard work for you. It will serve as a distraction for the Grave Warden Duelist and will most likely attack it first. While this is happening, you can attack the boss with your ranged magic attack spells or, if there is a window of opportunity, rush in and land a few attacks with your strongest melee weapon. Depending on the spirit, some have quite a large amount of health, so it won't go down that easily and its attacks deal quite a lot of damage and can knockback the Grave Warden Duelist. Successfully defeating this boss yields 1200 Runes and drops 1700 Runes as well as Banished Knight Engvall Ashes. After the battle, you can use the teleporter ahead to return to the entrance or just travel back by foot if you prefer to.