Elixirs are a health-restoring item that, in most of its appearances, completely restores the HP of its user. It is effectively the highest tier of health restoration consumable, followed by Concoctions, Vulneraries, and Herbs. In some games, they appear as rare items, while in others, they appear in some shops.
As exceptions to the above, in Fire Emblem Fates, the health restoration granted by an Elixir caps at 40HP, while in Fire Emblem Engage, the effect is capped at 30HP.
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 unlocks the ability to upgrade from Concoctions to Elixirs for either a specific unit or all units at the Tactics Academy.
Other Series
The Elixir is treated as a special item in TearRing Saga: Utna Heroes Saga, indicated by a black star that precedes its name [★Elixir]. In this game, the Elixir is capable of reviving a dead unit.
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.