Are a Beholder's Legendary Action Eye Rays Also Bound to the "Reroll Duplicates" Rule?

Question

This is a balancing question as I am currently in the middle of homebrewing a Beholder-esque monster and I'm trying to use the Beholder's stat block as a template for how my new monster's challenge rating is determined. I know that in the Beholder's stat block it makes sure that the monster does not roll its death ray three times in a row, making its effective damage output per round skyrocket.

Eye Rays. The beholder shoots three of the following magical eye rays at random (reroll duplicates), choosing one to three targets it can see within 120 feet of it:

This keeps the Beholder fair and balanced during its turns, but what about its legendary actions?

Eye Ray. The beholder uses one random eye ray.

A similar caveat is not given. Would this mean that, should the players be unlucky enough, a beholder can fire off a death ray three turns in a row, potentially adding an additional 165 damage per round on average?

Once again, I'm asking this in order to determine a monster's effective challenge rating so that I can apply that to a homebrew creation. Just want a better understanding of the calculations of it so I know whether my own creation is too strong for my party and so that I may run these types of monsters with random attacks more effectively in my campaign. Thank you!

Answer 1

You're reading it correctly, Death Ray could be fired every legendary action

Rules perspective: "Reroll duplicates" is part of the on-its-turn Eye Rays action specifically, not the legendary action that fires a single ray. "Uses one random eye ray" does not incorporate any of the rules specific to the multiattack Eye Rays, and there is no general rule for Beholders that prevents using a given eye ray more than once per round, it's a restriction that applies per Eye Rays action only.

Logic perspective: The reroll duplicates rule is there because each eye has a separate effect, and in the course of a single action, any given eye can only fire once. When operating off turn, nothing prevents it from reusing that eye.

Logistics perspective: The more tracking the DM has to do, the harder their job is. They generally try to minimize this. Requiring the DM to, every round, ensure that none of the up to six rays (from a set of 10) are repeated, when split across four independent "action cycles" scattered throughout the round, is just unpleasant. They didn't want to do that, so no tracking is needed.

So yeah, in theory, a Beholder could fire off a Death Ray four times in a single round, a 30% chance to do so on their turn, and a 10% chance for each legendary action they take. 0.03% of the time, they'll fire it four times in a single turn and give the PCs a really bad day (unless the PCs have good Dex saves, in which case everyone is fine, and the Beholder looks pretty silly). They're Beholders, they're fairly scary.