Assassinate. During its first turn, the warchief has advantage on attack rolls against any creature that hasn't taken a turn. Any hit the warchief scores against a surprised creature is a critical hit.
Sneak Attack. Once per turn, the warchief deals an extra 14 (4d6) 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 warchief that isn't incapacitated and the warchief doesn't have disadvantage on the attack roll.
Fey Ancestry: The warchief have advantage on Saving Throws against being Charmed, and magic can’t put it to sleep.
Innate spellcasting. The warchief's spellcasting ability is Intelligence (spell save DC 11). It can innately cast the following spells, requiring no material components:
At will: dancing lights
1/day each: fog cloud, cloud of daggers, invisibility
Multiattack. The warchief makes two double-bladed saber attacks.
Double-Bladed Saber. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 6 (1d10 + 3) slashing damage, and the target must make a DC 15 Constitution saving throw, taking 24 (7d6) poison damage on a failed save, or half as much damage on a successful one.
Longbow. Ranged Weapon Attack: +6 to hit, range 80/320 ft., one target. Hit: 7 (1d8 + 3) piercing damage, and the target must make a DC 15 Constitution saving throw, taking 24 (7d6) poison damage on a failed save, or half as much damage on a successful one.
Nomad's Endurance (Recharge 5-6). When the warchief takes damage, it can use its reaction to roll a d12. Add its Constitution modifier to the number rolled, and reduce the damage by that total.