An interesting situation happened last night during our 4th-level party's exploration of Chult, where wandering monsters really don't care about your party level or whether or not they are an appropriate CR. I'm not sure whether this specific encounter set-up was put together by the DM or comes from the adventure we are in, Tomb of Annihilation, so I will spoiler it:
We chanced upon a small group of zombies that appeared to be feeding on the carcass of a T. rex. We have a lot of ranged PC's, so several zombies we can handle. As soon as we attacked, however, the undead T. rex got to her feet and charged us, disgorging more zombies as it came.
We were thus facing a medium-sized group of low CR undead (hereafter minions) and one huge high-CR undead (hereafter boss monster). The boss monster was easily capable of taking any one of us down with a single hit. Trying to protect the rest of the party, the Crown Oath paladin used his Channel Divinity: Champion Challenge class feature on the boss:
Champion Challenge. As a bonus action, you issue a challenge that compels other creatures to do battle with you. Each creature of your choice that you can see within 30 feet of you must make a Wisdom saving throw. On a failed save, a creature can't willingly move more than 30 feet away from you. This effect ends on the creature if you are incapacitated or die or if the creature is more than 30 feet away from you.
The boss monster failed its save and obligingly charged the paladin. However, the minions came streaming in as well, threatening to take down the paladin even if he withstood the boss. The party cleric then moved behind the paladin and used her Channel Divinity: Turn Undead class feature on the entire group, boss and minions:
Each undead that can see or hear you within 30 feet of you must make a Wisdom saving throw. If the creature fails its saving throw, it is turned for 1 minute or until it takes any damage.
A turned creature must spend its turns trying to move as far away from you as it can, and it can't willingly move to a space within 30 feet of you. It also can't take reactions. For its action, it can use only the Dash action or try to escape from an effect that prevents it from moving. If there's nowhere to move, the creature can use the Dodge action.
Fortunately for us, most of the minions failed their saves. The boss made her save, which was good because it meant we did not have to stop the combat and figure out how to rule the two effects. However, I am still unsure what should have happened had she failed, and been simultaneously compelled to do battle with the paladin and flee from the cleric standing just behind him.
According to Champion Challenge, the boss could not willingly move more than 30 feet away from the paladin - but fleeing the turn effect is not willing movement, it is compelled. According to the Turn Undead, it must try to move away, and can't willingly move to a space within 30 feet of the cleric - but again, moving toward the paladin is not willing, it is compelled.
As far as I can make out, on a failed save the boss should have begun to move away from the paladin, but stopped when it was 30 feet from the paladin and 35 feet from the cleric, and then been forced to remain there, simultaneously unable to move further away or return closer. But I would appreciate independent confirmation or alternate explanations.
Champion Challenge is similar to Compelled Duel in that (as this question notes for Duel), it says it compels the target to attack the caster, but then never provides any mechanism for this to happen, and instead only explicitly affects the target's movement. If your interpretation of what would happen to the boss monster in this question depends on whether or not it is forced to attack, you should make that point explicit.
