This potion is absolutely crazy. Its healing ability is so high that even those who have it, never use it. You can't buy this anywhere, at least anywhere with sane people. The potion does the following:
Heals 20d4+40 hp.
Gives the drinker Regeneration for 10 minutes at 5 hp per round at the start of their turn.
If the player hits 0 and goes down after they had this potion but does not outright die, at the start of the characters next turn, as long as they still have the regen, they are brought back up to 50% of their max hp but they loose the regen from the potion regardless of how much time has passed since drinking the potion.