Demon Arts: Shadow Coffin

The caster recites an incantation, causing the sky to appear darker and the gravity in the area to increase. Caster takes 4d10 Necrotic damage. The target must make a Dexterity saving throw as the shadows in the area begin to solidify and form a box around the target. On a fail, the target takes 4d10 force damage and is paralyzed inside the box. On a successful save, they take half damage and avoid getting trapped. 

The target inside the box takes 4d10 Force damage per turn unless they succeed 3 saves. Inside the box is magical darkness and silence. The box has an Ac of 23 and a Hp of 50. While inside the box, those trapped are immune to any damage dealt from outside the box. They must make a Wisdom saving throw each turn. On a success, they take no damage. On a fail, they take 4d10 force. If they fail 3 saves they remain paralyzed until the caster loses concentration, the spell ends, or the box is destroyed. If they succeed 3 saves or roll a Natural 20, they break free of the oppressive force and may act normally and attempt to destroy the box from within. If an individual is trapped for the entire duration of the spell or their health reaches 0, the box begins to crack before slowly fading, killing them in the process. The dark clouds in the sky vanishing, leaving a clear sky. 

 The spell can increase by 1d10 Force damage per level above 7th.