The Forever Compund is an alchemical serum produced by the Brotherhood of the Shield. It is a diluted form of the Elixir of Immortality developed by Isaac Newton in 1652.
While it still retains the life-prolonging properties of its parent elixir, it doesn’t keep any of the other properties. In particular, it does not prevent the mind or body of the person injected with it from decaying.
Newton administered it to Nostradamus in 1652, which kept the seer alive and also kept his prophetic gifts intact until Nostradamus took the Elixir of Immortality three hundred years later.