Unlimited Imprisonment (無限牢獄 mugen rōgoku) is a Unique Skill of the Nameless Hero that entombs the target in a complex number of spatial dimensions and also seals away most Skills within. It does not weaken over time. It cannot be damaged via standard Physical Damage.
This Skill was used by the Nameless Hero to seal the Storm Dragon Veldora. The Unlimited Imprisonment remained active for about 300 years until Rimuru Tempest shallowed it whole alongside Veldora. Some time later, Rimuru' Unique Skill Great Sage evolved into Ultimate Skill Raphael while Veldora's Unique Skill Investigator evolved into Ultimate Skill Faust - and thanks to these Skill evolutions the analysis time of the Unlimited Imprisonment got reduced from about 100 years to just 1 day. As a result, Rimuru obtained Unlimited Imprisonment himself, which was then integrated into Ultimate Skill Uriel.