Jarvisian Life Elixir
The Jarvisian Life Elixir was a potion created by Jarvis. It dramatically decreased the effects of ageing in humans. Only three people are known to have taken it: Jarvis himself and two of his friends in his early life: Kensa and Etende. It worked, as the three who had taken it had only seemed to have physically aged around five years while a hundred passed. Only one batch was known to exist, the remains of which were stolen at gunpoint by a hooded figure. Jarvis never wrote down the formula, so the key to creating it died with him.
Jarvis claimed that it would be deadly to most non-human forms of life, but as it had not been seen tested on other forms of life, there is no way of knowing this for sure.
Canonically, the only known batch of the elixir was consumed sometime between the events of Heat the Soul and the three's reappearance in the main RPG.