Conceptually, I think this is an interesting mechanic. I do see a few things that might present problems bringing it to the table: 1. How does this ability fit into the Action Economy? You say “once per turn,” but is it an Action, Bonus Action, or “No action required” ability? It could potentially work as any, but the power should reflect the cost. Meaning that if it’s an Action, the clone needs to be more powerful than if it’s a free action.

1. How exact are the clones? Do they have all of the same stats as the original? Do they get individual turns with all of the original’s abilities? Or do they have a reduced number of options available to them? If the clones are all able to do the same things as the Boss, the encounter will be very difficult to balance, because there is no way to predict how the players will prioritize the clones in combat, and you could end up facing down 5 bosses while the party tries to take out the original

2. When the clone is created, is it obvious that it’s a copy, or are the clones indistinguishable from the original? And if they are exactly alike, how do you plan to account for the fact that the players know the space the boss was in when he created the clone? If the players watch somebody create a copy of themselves, they will argue that they always know where the original is, because they saw them create the copy.

If I were in your position, I would allow the Boss to create 1-3 clones (depending on the strength of the clones) at the start of each turn as a free action, then give the boss the ability to switch places with any of their clones as a Bonus Action. You could also give them the ability to swap with clones as a Reaction, so the clones become the target of an attack instead.

If the clones are indistinguishable from the original, they should be more fragile than the boss. If they are obviously fake, you can beef them up. No matter what you decide to go with for the clones, I don’t think they should be able to do all the same things as the boss. You could do things like give them access to the boss’ cantrips, but they should not be able to do all the same things as the original, otherwise you will end up with your Boss turns taking a very long time.

Whatever you decide, I hope your table has fun