Disclaimer: I know that monster balance is never exact and I don’t expect it to be, I’m looking for rough guidelines like they are given in the DMG for many traits.
I like to flavor up my monsters with spells. One spell that seems particularly interesting is banishment.
Monsters with that spell appear in the MM, e.g. the Death Knight, Sphinxes, and Guardian Naga. It is not clear however what effect the spell had on their CR.
I came up with the following assumptions:
It seems that since that since banishment is 4th level, i.e. for level 7 players, it should not appear below level 7 encounters.
Further, the spell requires concentration, but it seems reasonable to assume that a boss monster (no banishment for random critters) can hold concentration for some time through good saves, defense from minions, and, potentially, legendary resistance.
The effect of a missing PC is relevant for offense and defense since the party’s damage output is reduced and the remaining PCs get more damage (relatively).
a priori it does not seem that banishment will get much better or worse on higher levels, since both the concentration maintaining capacity and the effect of a missing person seem to scale with level.
Are these assumptions valid? And how can these (and other factors that I might have missed) translate into CR?