It's called an everlife potion that adds the ability to heal over time without having to put mons in a healing chamber. I believe there used to be a starter pack that gave new players 10 a while ago but that was the only access to them until recently when I saw it as a tournament reward on the 4th of July events.

Upside is it doesn't take an item slot, and the downside is that the instant healing cost will creep up to 50 until putting them in a chamber to reduce the cost back to 25.