Black Gulch
Prior to exploring the Black Gulch, visit Melentia and ensure you have purchased all three Brightbugs from her.
25. Black Gulch, Dark Souls II: SotFS
The Black Gulch is another blighted area similar to The Gutter, but the Black Gulch is lined with poison spitting statues. Prepare at least 30 Firebombs and have them ready in your inventory. This area has many more oil pools than at The Gutter. Instead of Firebombs, you might want to use a bow and Fire Arrows to set the oil pools ablaze. Also have plenty of Poison Moss and stock up Lifegem x 99. As with The Gutter, use a secondary weapon such as the Whip to break the poison spitting statues. Slowly make your make through the area, breaking poison statues as you go. Two NPC red phantoms will invade you, one at a time: Woodland Child Victor and Woodland Child Gully. Defeat one NPC invader, quit to menu, and load your game again to trigger the other NPC red phantom to invade you. Summon Bashful Ray and Lone Hunter Schmidt to help you gank each NPC invader. Lucatiel can also be summoned, but after you have located her, exhaust her dialogue, and unlock the Hidden Chamber bonfire. The bonfire is blocked by a petrified enemy.
There are two locations where you can safely drop off a cliff; use the torch to help you find the locations. One drop off point leads to the two Elite Giants whose defeat would reward you with the Soul of a Giant and the Forgotten Key. The Elite Giants are vulnerable to poison. Hide yourself inside a cave and use Poison Arrows and/or Poison Knives to poison the giants. The Soul of a Giant will be useful during a late-game boss fight, while the Forgotten Key will be used to unlock a room containing an item that allows you access to one of the DLCs. The key can also be used to access the room at the Black Gulch where Darkdiver Grandahl is located. Following Grandahl's quest line would allow you to fight an optional boss. The other drop off location leads to Lucatiel. Locate Lucatiel inside a tunnel and exhaust her dialogue to advance her quest line.
The Rotten
Use a weapon that has a thrust attack and buff your weapon with lightning. Aromatic Ooze and Charcoal Pine Resin are also effective, but Dark Pine Resin should only be a last resort. To progress in Lucatiel's quest line, you must summon her for this fight and ensure she survives the fight. This should be the third, and last, time that Lucatiel helps you against a boss. If she survives all three (different) boss fights, then you have mostly completed Lucatiel's quest line. You will meet her for the last time at a later area of the game, thus wrapping up her quest line.
Defeat The Rotten, light a Primal Bonfire, and visit Melentia to purchase all Brightbugs from her.