I am running a collection of one shot adventures for my group. In one, the party's fighter was deemed impressive by the Lord of the Hunt, who gave him a spear designed to send powerful creatures to him for the Wild Hunt. 

What I have planned is a once a day effect where the target (which must be a beast, monstrosity, or something similar) must make a Charisma saving throw or be sent to the domain of the Lord of the Hunt. The target automatically fails if the spear's damage reduces the target to 0 hp. What would be a good system for setting the DC? Based on monster hp?

I’m not entirely sure I understand correctly. Could you please give us some more details of your intentions for this magic item?

Basically, it's a side quest of sorts where the fighter would "capture" powerful monsters for the Wild Hunt by using the spear. The more dangerous the monster, the better the reward. 

I would have it so that that weapon had a special property that could be used once per day on a hit, that if the total damage rolled for the attack would equal or exceed the creature’s current HP the monster gets teleported to Wild Hunt instead of killed. Something like that. Make sense?

It's close to my original idea of a saving throw automatically failing. The big issue there is that the fighter would have to use guesswork to know when to switch from glaive to spear (he has Polearm Mastery) to make the final strike.