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!