“A powerful solution that restores all HP.” —In-game description, The Sacred Stones
Elixirs are HP-restoring items that appear across the Fire Emblem Series, beginning with Fire Emblem: The Binding Blade. In line with its etymological role as a magical restorative that extends a given person's life when consumed, the Elixir is capable of completely restoring the HP of its user. This effectively makes Elixirs the best HP-restoring item, followed by Concoctions, Vulneraries, and Herbs. Given the massive benefit that the Elixir brings, it comes as no surprise that it is rather scarce, usually accompanying characters who join at a juncture nearer the Endgame point.
The Elixir is regarded to be a special item in TearRing Saga: Utna Heroes Saga, a fact that is indicated by a black star that is assigned to its name [★Elixir]. In this game, the ★Elixir's etymological role is further emphasised on, as it, contrary to its Fire Emblem counterpart, possesses the effect of resurrecting its user when they run out of HP.
In Fire Emblem Warriors: Three Hopes, Elixirs are an upgrade for a playable unit's healing tonics. To unlock it, one must first unlock the effect "Research Advanced Medical Arts" in the Facility Manager at the Base Camp, which then unlock the ability to upgrade from Concoctions to Elixirs for either a specific unit or all units at the Tactics Academy.
Para. 5 - Speak to the Villager located towards the northern reaches of the map. Para. 6 - Provided that Inigo succeeds in killing at least one enemy unit.