Use Simple Alchemy in the Processing window (L) to combine Mandragora Essence x1, Essence of Magic Power x1, Oil of Regeneration x1, Troll Blood x2, Grape x4 into Elixir of Regeneration.
It can be produced by using Alchemy Tool in your residence if Apprentice Lv. 1 or up in Alchemy.
> From an Alchemy Tool in your residence if your Alchemy is Skilled Lv. 1 or up.
It can be obtained by Farming Grape Seeds in a Garden or by Gathering from Grapes using Bare Hands or a Hoe.