The water from the mythical Fountain of Youth! Probably. However the healing properties of this Shadowrealm liquid are not a myth. Each gulp of it restores 25 HP, 50 Stun Damage, 50 Morale, 75 Energy and 10 Freshness.
The phenomenon is not explainable within our laws of physics; in contact with inanimate matter, the liquid behaves like normal water. What is important, though, the Water of Life retains its properties in our reality, as long as it stays relatively unchanged.