Darklurker

The Darklurker is an optional boss whom you fight after clearing three dungeons. Clearing the dungeons and defeating the Darklurker is the quickest way to obtain the Selfless Giver Steam achievement.

Dungeon 1: Shaded Woods

80. Dungeon at Shaded Woods, Dark Souls II: SotFS

Start from the Shaded Ruins bonfire and make your way to Darkdiver Grandahl, who is located within an underground cave filled with various Basilisks. Exhaust Grandahl's dialogue. This should be the first time that you talk to him.

Dungeon 2: Black Gulch

81. Dungeon at Black Gulch, Dark Souls II: SotFS

Start from the Hidden Chamber bonfire. Light your torch and walk along the cliff, finding the location where you dropped off to reach the two Elite Giants. Grandahl's location is the ledge atop the cave where you fought the two Elite Giants. Use the Forgotten Key to unlock the stone doors leading to Grandahl and exhaust his dialogue. This should be the second time that you talk to Grandahl.

Dungeon 3: Drangleic Castle

82. Dungeon at Drangleic Castle, Dark Souls II: SotFS

Start from the Under Castle Drangleic bonfire. Light your torch, find a Dark Stalker nearby, and defeat it. Then exhaust Grandahl's dialogue. This should be the third time that you talk to him. Grandahl now offers you the chance to join his covenant, i.e. the Pilgrims of Dark covenant. Join the covenant and a portal would open, but don't use this portal yet.

Revisit Grandahl at Shaded Woods and offer him a Human Effigy so he would open the portal at Shaded Woods. Interact with the portal and be transported to the dungeon at Shaded Woods. Defeat all enemies within the dungeon, light a sconce, make your way to the end of the dungeon and exit. In case your character dies while in the dungeon, you must offer Grandahl another Human Effigy for him to open the same portal. Next, revisit Grandahl at the Black Gulch. Offer him a Human Effigy, he opens up a portal, interact with the portal, and be transported to the dungeon at the Black Gulch. Your objectives while in this dungeon is the same as per the dungeon at Shaded Woods. Finally, revisit Grandahl underneath Drangleic Castle. The portal at Drangleic Castle should already be open. Before entering the third dungeon, prepare your character for fighting the Darklurker. Interact with the portal and be transported to the dungeon at Drangleic Castle. Complete the same tasks as per the other two dungeons. After completing all objectives within all three dungeons, exiting the third dungeon would transport you to the arena of the Darklurker, the boss of all three dungeons.

Darklurker

83. Darklurker, Dark Souls II: SotFS

This boss is weak against lightning. Use the strongest lightning miracle you have or buff your melee weapon with Gold Pine Resin. The Darklurker deals only elemental damage. Stack as high fire, dark, and magic defense as you can. This is a boss that can only be fought solo. No summons are allowed, NPC or otherwise.