Avada kedavra

As you gather courage to say the forbidden words, you hold out your hand, and say the words "AVADA KEDAVRA". You see as a beam of green light shoots out of your hand, homing after the creature you pointed to. The creature must succed a DC 20 dex save. On a failed save the creature takes 10D10 Damage. On a succeded save the damage is halfed. When a creature is hit with this spell every roll against it has advantage, and everything that it does has disadvantage. this lasts for 3 rounds.