In the first game, the player receives the Estus Flask from Oscar, Knight of Astora in the Northern Undead Asylum. Initially, players can only replenish five uses by resting at bonfire, or ten by resting at a bonfire tended by a Fire Keeper. Players may increase their uses by kindling a specific bonfire. Kindling requires being in human form and holding one humanity. A bonfire can be kindled up to three times (once the Rite of Kindling has been obtained from Pinwheel), increasing replenished uses by five each time for a maximum of twenty replenished uses. This kindling is permanent and carries over into NG+. Fire Keepers may also reinforce the flask for the player so it recovers more health with each use, but only if he or she gives them a Fire Keeper Soul to complete the process. In addition to finding these special souls in the environment, each Fire Keeper drops a unique Fire Keeper Soul. The living Fire Keepers found in the game are Anastacia of Astora, Lady of the Darkling, and Quelaag's Sister. The most a flask can be upgraded is seven times.
Dark Souls 2
Estus flasks work very differently in Dark Souls 2. Unlike Dark Souls 1, the health replenishment is not immediate but gradual. Upgrading the flask and its number of uses is also different. Instead of giving firekeeper souls to other firekeepers, burning Sublime Bone Dust in a bonfire increases the amount of HP restored. Uses are no longer bonfire-specific but dependent on another upgrade item. To increase the uses of the flask, one must present an Estus Flask Shard to the Emerald Herald in Majula.