Timeless Tomb

A rift in the Material Plane opens beneath the target and the pure cold of Stygia envelopes them. The target must make a Constitution saving throw, taking 5d12 Cold damage on a failure, and half as much on a success. If the target dies or it's hit points are no more than 5 times the spell's level, the target is encased in unmelting ice, where it remains in stasis until the Tomb is destroyed. 

The Tomb is always one size above the targeted creature. While the target is entombed, they are Blinded, Deafened, Restrained, Incapacitated, Unconscious, they do not age nor regain hit points and they are immune to any damage or conditions after being entombed. The Tomb freezes the ground around it, making a 15-foot circle of difficult terrain. The Tomb has a number of hit points equal to 3 times the amount of damage the target took from the spell. The Tomb is immune to all damage except for Blunt, Piercing, Thunder, and magical Fire. If attacked by a cold spell, the Tomb's hit points increase by half the number of damage the attack dealt. If the Tomb is reduced to 0 hit points, it is destroyed and the spell ends, meaning the target is free. 

At Higher Levels: If the spell is casted at 6th level or higher, the damage die increases by 1d12 for every level above 5th level.