Estus Flask

Estus Flasks are a consumable item in Dark Souls, allowing the Undead to replenish missing health. 

The Undead treasure these dull green flasks. 
 Fill with Estus at Bonfire. Fills HP. 
 The Estus Flasks are linked to the Fire Keepers. The Dark Tales also make reference:

An emerald flask, from the Keeper's soul. She lives to protect the flame, andand dies to protect it further.

Usage

Estus Flasks can be consumed by placing them either in the player's quick access (highly recommended) or navigating to the flask in the inventory. They have a limited number of uses, indicated by an overlaid number, and can be replenished by resting at a bonfire. The number of Flasks returned when resting at a bonfire depends on its level of kindling, starting at 5 flasks, then increasing to 10 when the bonfire has been kindled and 15/20 once the player has acquired the Rite of Kindling and can kindle the bonfire further. It costs the player 1 humanity point to kindle a bonfire.

Additionally, drinking an Estus Flask will heal summoned players in co-op, but not NPC co-operators. 

Acquisition

Granted to the player at the start of the game as an item found on the corpse of Oscar of Astora. If not picked up in NG+, can be found in the chest behind Frampt in Firelink.

Upgrades

Can be upgraded with a Fire Keeper Soul up to +7 in order to restore more HP per sip. A Fire Keeper Soul must be given to one of the three Fire Keepers, Anastacia, The Fair Lady, or the Darkmoon Knightess.

Estus Level	0	+1	+2	+3	+4	+5	+6	+7
HP healed	300	400	500	600	650	700	750	800