Infinite Regeneration

Endless Regeneration (無限再生) is an Intrinsic Skill that has been granted to Rimuru Tempest by the Voice of the World during his Harvest Festival ascension to a True Demon Lord. Specifically, he gained it for his evolution from Slime to Demon Slime.

Endless Regeneration is vastly superior to the previous regenerative Skill Rimuru had, Ultraspeed Regeneration. While Ultraspeed Regeneration becomes disabled by Spirit Particles, Endless Regeneration under those conditions by increasing the Magicule input. That means Rimuru would be able to withstand at least one Melt Slash or the like while standing still.