Assassin, Variant (5e Creature)
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Medium humanoid (any race), any non-good alignment, 12th-level rogue
Saving Throws Dex +8, Int +5
Assassinate. During its first turn, the assassin has advantage on attack rolls against any creature that hasn't taken a turn. Any hit the assassin scores against a surprised creature is a critical hit.
Evasion. If the assassin is subjected to an effect that allows it to make a Dexterity saving throw to take only half damage, the assassin instead takes no damage if it succeeds on the saving throw, and only half damage if it fails.
Sneak Attack (1/Turn). The assassin deals an extra 21 (6d6) damage when it hits a target with a weapon attack and has advantage on the attack roll, or when the target is within 5 feet of an ally of the assassin that isn't incapacitated and the assassin doesn't have disadvantage on the attack roll.
Shortsword. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 4) piercing damage.
Uncanny Dodge. The assassin halves the damage of one attack that hits it.
See 5e SRD:Assassin.