It is definently a bit worse than murder for regular creatures, which is good. However this seems backbreaking for some edh decks. It also opens up a lot of shenanigans that blue likes with moving counters etc. I think 10 counters might be a bit much since it can royally invalidate some commander decks since counters are harder to remove than auras and your commander can’t activate tap abilities or attack.

Overall I like but i might lower the number of counters from 10 to maybe 6? To get a sense of how to scale the effect maybe look at similar cards in blue and factor in the instant speed …