An elixir which was saught by alchemists in the past.

The elixir would either bring immortality, cure disease, or bring one back to youth.

One example of such an elixir: Po-Yang's:
Gold fluid is cooked for thirty days, and then mixed with quicksilver and placed in a yellow jar, which is then sealed with mud and heated for sixty hours. The resulting "medicine" will give the taker immortal powers.