Vicious Ice Bolt

You create a stake of ice and shoot it at one creature within range. Make a ranged spell attack against the target. On a hit, the target takes 1d10 piercing damage. On a roll of 20 or higher, the target takes an additional 2d6 piercing damage. The target must succeed on a Constitution saving throw. On a fail, the target takes 2d6 cold damage, movement is halved until the end of their next turn, and the next attack or saving throw is reduced by 1d4. On a success, the target takes half of the 2d6 cold damage.

To add this spell to your spell list, the caster must cast Ice Knife or Ice Bolt on a target a total of 10 times. Then after 8 hours of studying the book they must make a DC 18 Arcana check.