Creating Elixir of life, Nectar/Amrita through alchemy. Not talking about just physical immortality or eternal youth.
Read in a LHP book about a French prostitute over 70 years who still looked like in her mid 30’s cuz she would eat/drink five men’s cum everyday.
Inner alchemy of taoism can give both absolute immortality , make one rise above food and drink. And a Nirmanakaya body, that is when ever one wants he can manifest a physical body from higher realm and after working dissolve it instantly.
Indian alchemy of Shaivism which is highly based on purified mercury can give absolutely immortal/non destructible physical body of flesh and blood.
Now all this are considered to be inferior to the effect of drinking true nectar/amrita/elixir, which is even sought by the cosmic gods. Now what’s the recepie of making it, well I don’t know. But in every occult tradition it is considered to be the highest practice, goal of alchemy, not just turning metal/stone into gold.