As you say the incantation "Avada Kedavra" a jet of green light flashes from your spellcasting focus. Make a ranged spell attack. On a failed attack, the spell does not manifest in any meaningful way. The target also rolls a DEX save to avoid the spell. If the ranged attack hits AND the target fails their save, the target is instantly killed, regardless of any resistances or immunities. As a reaction, any of the target's allies within 10 ft. can block the spell with their bodies, eliminating the DEX save. On a hit, that ally is killed instantly and the target gains immunity to all negative effects from any spell you cast, including Avada Kedavra. When targeting a huge Huge or Gigantic creature, you have disadvantage on the attack roll. At level 20, you no longer have this disadvantage. Good-aligned characters cannot use this spell unless allowed by the DM under special circumstances.
* - (A spellcasting focus)
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