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, and dies to protect it further.