- In Undead Parish, on an altar in the Undead Church, guarded by a Berenike Knight.
- In New Londo Ruins, it can be seen on the right hand side when crossing the narrow wooden walkway. Three ghosts will spawn when getting near it.
- In Blighttown, in a waste-water pit accessible after taking the wooden elevator, guarded by Flaming Attack Dogs and Blowdart Snipers.
- One is located in The Duke's Archives inside Big Hat Logan's prison. Access granted by the Archive Tower Giant Cell Key that is acquired later in the area. Not to be confused with the Archive Tower Giant Door Key.
- Obtained after killing Quelaag's Sister, which eliminates the ability to join the Chaos Servant covenant on the current playthrough and extinguishes the local bonfire.
- Obtained after killing Darkmoon Knightess at the first bonfire of Anor Londo. Killing her extinguishes the bonfire.
- Obtained after defeating Knight Lautrec of Carim through the Black Eye Orb in Anor Londo. Players may choose to either return the soul to its owner, restoring the Firelink Shrine bonfire; obtain humanity from it; or offer it to another Fire Keeper, increasing the power of their Estus Flasks.
- The typical use of a Fire Keeper Soul is to give it to one of the three Fire Keepers: Anastacia of Astora below Firelink Shrine, Quelaag's Sister in Quelaag's Domain, or Darkmoon Knightess at the main bonfire in Anor Londo to reinforce the Estus Flask, increasing its healing powers per charge.
- If used from the player's inventory (though not recommended unless the flask is already fully reinforced), a Fire Keeper Soul will grant the user with 5 units of liquid Humanity and restore full health. The consuming animation is significantly faster than that of a regular humanity item.
- The player may not have duplicates of a Fire Keeper's unique soul (obtained by killing an in-game Fire Keeper), meaning only seven can be carried at a time. Thus, it's good to make a habit of offering them to Fire Keepers whenever one finds them until the Estus Flask is fully reinforced.
- The Estus Flask can only be reinforced a total of 7 times, meaning that only 7 Fire Keeper's souls may be used in such a manner. Any subsequent souls obtained (only possible past NG as each playthrough yields a maximum of 7) may as well be consumed.
- Anastacia's Fire Keeper Soul can be used to revive her, allowing the player to re-access the Firelink Shrine bonfire and speak to her. If her soul is used instead, the fire can never be re-lit until the next playthrough.
- As it requires an active Fire Keeper to reinforce the player's Estus Flask, it is not possible to obtain +7 Flasks on one playthrough.
- Along with Gargoyle Tail Axes, Fire Keeper Souls are the only items with identical icons and multiple in-game descriptions.