Estus Flask (5e Equipment)
From D&D Wiki
Wondrous Item, rare (requires attunement)
A small flask made of pale green glass that holds glowing, golden liquid.
A creature can use its action to take a swig of the liquid in the flask; when it does, it expends one of the flask's charges, to roll one of that creature's hit dice, regaining hitpoints equal rolling the hit die plus the creature's Constitution modifier as if expending hit dice during a short rest. That creature can also use its bonus action to expend another charge to roll another hit die (after having used its action to drink from the flask first). If that creature's hitdice pool is empty, however, it instead only regains hitpoints equal to only its Constitution modifier (with a minimum of 1) and gains one level of exhaustion.
The flask's liquid tastes like hot tea, but has an oddly coppery aftertaste. Bizarrely, its effect works on undead creatures as well as living creatures, which can even taste it, regardless of whether or not they should be able to do so (or even of whether or not their tongues are still intact).
The liquid in the flask vanishes if it is poured out without being drunk by a creature, and mixing it with any other substance dilutes its effect, such that any drinking creature will be affected as if that creature's hitdice pool were already empty, in addition to losing one hitdie if it had any. This dilution lasts until the foreign substance is completely gone from the flask.
The flask contains liquid only as long as it has charges, and holds up to five charges. It regains these charges whenever you finish a long rest, but only if the flask is directly next to an open flame that is large enough to provide warmth to the creature (such as a typical campfire or fireplace).