Can be used to increase your Estus Flask level to +1, to a maximum of +7, meaning each use of Estus Flasks will heal a larger portion of your health, by giving it to Anastacia of Astora under the Firelink Bonfire, or any other Fire Keeper such as the Witch in Quelaag's Domain, or in Anor Londo.
In the Undead Parish, on an altar shortly before the Gargoyle fight
This one isn’t until past the giant metal boar. Once you make it to the church with the bell tower(also the building on which you fight the Belltower Gargoyles) you will see an alter and a large knight protecting it. This guy gives the illusion or being strong. Bust out the Drake Sword and go towards him. All he will really do is try to hit you with his sword and block your attacks. Dodge his sword swipe and hit him a few times with the sword to bring him down with little effort. Grab the item he is protecting and you can upgrade your Estus Flasks again. If you don’t take the steps up in the church and keep going you will see an elevator. Take that elevator and you will be closer to the person who can upgrade your flask as well. Don’t worry you can take it back up afterwards.
New Londo Ruins
In New Londo Ruins, shortly after crossing the second bridge
This Fire Keeper Soul is fairly early in the game. After the crow drops you off near the fire you should see a guy standing near it. To the left off him are some stairs that are hard to see. Head down but watch your footing it is very dangerous. At the bottom you will see a cage. In that cage is the person who will upgrade your flask. Keep following the path down until you step on a switch and an elevator takes you down.
You will now be in the ruins. If you go left you should see some water and a bridge you can cross. Cross the bridge but be careful once you get to the end because of the ghosts. You will need to use Transient Curse or magic on them to bring them down. You can run past them if you want but they will chase you.
Where you find the first two ghosts is where we want to be. If you go right in the shallow water and keep looking back towards the way you came you should see the glowing orb on the ground. You will quickly see the path to it. It is a very narrow route and once wrong step means death so take your time. Once you get there be sure to grab the item and be ready to fight. Three more ghosts will spawn and you can kill them if you are able or die but still have the soul. After you get that bring it back up to the caged person and get the upgrade!
Blight Town/Quelaag's DomainEdit
On the left-hand side of the map it is the blue marker directly next to the two red Xs. In a concrete drain-like area, it is surrounded by blow dart snipers and the red, fire-breathing dogs.
Approaching Blight Town from the direction of the Valley of Drakes will mean a shorter travel distance.
Duke's Archives/Crystal Cave
In the giant cell with Big Hat Logan, guarded by the blue octopus head guys. You need the Archive Tower Giant Cell Key to open the door. It is found later in Duke's Archives.
After you defeat Chaos Witch Quelaag, you can go down into the demon ruins and strike a wall to reveal a bonfire. Next to the bonfire is a harmless, heavily mutated fire keeper (a bit hard to spot as it appears to be a wall, but you will spot which wall it is easily enough) that will die in a few hits and drop his soul. Killing it will make the 10-flask bonfire unusable, so it is recommended you beat the Ceaseless Discharge before doing this.
- Knight Lautrec of Carim
- Lady of the Darkness
- Daughter of Chaos