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Round 2: Tyekanik

Tyek easily blocks the hag's attacks, laughing in his lust of battle.  Seeing that the Ashen Rider has not fallen, he directs his first attack(s) at that foe, hoping to kill it, leaving just the Night Hag and the Amnizu fiend.

Movement: None.  Holding his ground and preventing their opponents from reaching the party members at the east end of the chamber.
Action: Two Reckless Attacks with Annam's Greataxe as his Elemental Cleaver (thunder)
  Attack 1 on the Ashen Rider: 30, Damage: 28 (thunder) + 24 (force) = 52 (total)
  Attack 2 on the Ashen Rider or, if it's now dead, on the Night Hag: 34, Damage: 26 (thunder) + 18 (force) = 44 (total)
Bonus Action: if either of the attacks was a critical hit, or if Tyek killed the Ashen Rider, the Great Weapon Master feat allows him to make an extra attack as a bonus action.
  GWM Bonus Attack on the Night Hag: 32, Damage: 29 (thunder) + 18 (force) = 47 (total)

Since we're fighting undead and Tyek is within 30-ft of Khessa, the opponents' attacks against him should be a straight attack roll. (Disadvantage from Khessa's sacred idol's Prot. from Evil cancels out Advantage on attacks from Tyek's use of Reckless Attack.)

 Your first strike finishes off the ashen rider, the other 2 hits the hag

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Round 2: Vrox

Vrox swings out from behind the column and fires his longbow, first at the Night Hag and then at the Amnizu before moving to a new hiding spot.

Movement: Moving along the east wall of the room to get a clear shot at his targets and then moving to find a spot where he can hide from them again.
Action: Two attacks with Dragon's Wrath Longbow (150/600)
  Attack 1 on the Night Hag: 36 Damage: 13 (magical piercing) + 24 (radiant) + 6 (psychic) = 43 (total)
  Attack 2 on the Amnizu: 33 Damage: 10 (magical piercing) + 6 (radiant) + 5 (psychic) = 21 (total)
Bonus Action: Hide - Stealth: 24 (using Vanish feature, advantage from Shadowfell Brand Tattoo)

 Both attacks hit