Slow A5E

You alter the flow of time around your targets and they become slowed. On a successful saving throw, a target is rattled until the end of its next turn.

In addition, if a slowed target casts a spell with a casting time of 1 action, roll a d20. On an 11 or higher, the target doesn't finish casting the spell until its next turn. The target must use its action on that turn to complete the spell or the spell fails.

At the end of each of its turns, a slowed target repeats the saving throw to end the spell's effect on it.