I'm planning to use two of this monster in a game. They're immune to frightened and I know the cleric loves to use turn undead. If he uses turn undead on it then it's just incapacitated I guess? Does it still run away?

>Turn Undead. As a Magic Action, you present your Holy Symbol and censure Undead creatures. Each Undead within 30 feet of you must make a Wisdom saving throw. If the creature fails its save, it has the Frightened and Incapacitated conditions for 1 minute. For that duration, it tries to move as far from you as it can on its turns. This effect ends early on the creature if it takes any damage, you have the Incapacitated condition, or you die.