The description of the Sickening Radiance spell says (XGtE, p. 164):
When a creature moves into the spell’s area for the first time on a turn or starts its turn there, that creature must succeed on a Constitution saving throw or take 4d10 radiant damage, and it suffers one level of exhaustion and emits a dim, greenish light in a 5-foot radius.
The grammar and punctuation here makes me wonder: do the level of exhaustion and greenish light happen regardless of the result of the saving throw, just like how some spells still deal half damage when you succeed on the save?
I suspect they do. Otherwise it should have been written “…must succeed on a Constitution saving throw or take 4d10 radiant damage, suffer one level of exhaustion, and emit a dim, greenish light…”
Interestingly enough, the current description of the sickening radiance spell on D&D Beyond says:
When a creature moves into the spell’s area for the first time on a turn or starts its turn there, that creature must succeed on a Constitution saving throw or take 4d10 radiant damage, suffer one level of exhaustion, and emit a dim, greenish light in a 5-foot radius.
However, this wording change is not mentioned in any official errata (there has only been 1 errata PDF for XGE).
Wizards of the Coast has created changes between print versions before that were not included in official errata releases. D&D Beyond, as an official digital partner, may have already gotten a revised version even if a new printing has not been made for the physical book.
…Or it could be a typo on their website.
Unfortunately, I do not have access to the most recent printing of XGE. Anyone who does, feel free to comment and let me know if it has changed.