Arcagh can be found on a battlefield just west of the Selûnite Outpost among other Drow in the same condition.
During the fight with the Spectator he can be unpetrified by the monster and attack the party.
If he's unpetrified after the fight, then the situation can be resolved peacefully.
Normal weapon damage + Superiority DieWeapon damage
Focus your attack on your foe's hands and possibly force them to drop the weapons they are holding.
Normal weapon damage + Superiority DieWeapon damage
Spend a superiority die to deal an additional 1d8Physical damage and possibly push the target back 4.5 m / 15 ft.