Question about a new ability or buff/debuff that was added in this new season

Hi all, I've been asking my teammates in several blind pick games but never got an answer. I played Lux about 2 weeks ago and something transformed me into some kind of green wisp/green phantomic ball- or maybe it was blue, reducing my vision radius to a very short range, and I think, made my enemies lose target on me or gave me stealth for a short duration. I can't point out what it was. Any ideas ? :)