Fey Wanderers get an ability called Beguiling Twist which reads:
(…) whenever you or a creature you can see within 120 feet of you succeeds on a saving throw against being charmed or frightened, you can use your reaction to force a different creature you can see within 120 feet of you to make a Wisdom saving throw against your spell save DC. If the save fails, the target is charmed or frightened by you (your choice) for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a successful save.
Let's say that someone used a fear effect in round 1 and the target succeeded on the save. The Fey Wanderer used Beguiling Twist against Enemy B and Enemy B failed the saving throw. In subsequent rounds, when Enemy B eventually succeeds on the saving throw they get at the end of their turns, could the Fey Wanderer trigger Beguiling Twist again granted that their reaction is available?
I see no reason why it shouldn't work this way, but I've never heard it talked about and I've seen some guides talk about Beguiling Twist as if it doesn't trigger itself.
Best Answer
The text is ambiguous, up to the DM
Here the relevant text is the following :
The core of the issue here is the word "being". A "saving throw against being charmed or frightened" can be read in a few different ways :
Depending on which reading you believe is the most correct one, this ability is drastically altered.
I could not find any proof as to whether either of those interpretation is more or less "correct" in English language, so if you're the DM you'll have to pick the one you believe makes the most sense, or is the most balanced for your campaign. If you're the player, I recommend asking your DM, at the very least to avoid a last-minute bad surprise.