Frozen Dagger (Patreon)

Weapon (dagger), uncommon (requires attunement)

Attacks made with this dagger deal cold damage instead of piercing. If you roll a 20 on an attack roll made with this magic weapon, the target must succeed on a DC 12 Constitution saving throw or be paralyzed until the end of its next turn, frozen in place and unable to move. If it succeeds, its movement speed is halved until the end of its next turn instead. 

At first he thought the sudden chill was a simple, paralyzing fear of the dark.

Only then did he realize how wrong he was.