Question About Some Kind of Buff I Saw On An Opponent But Can't Figure Out

So my friend and I were playing joust and queued against an Izanami. Nothing out of the ordinary, but around level 12 I couldn't kill her anymore. I'd get some hits on her, but then they'd use something and her health bar would go blue where the lost damage was and she'd be immune to any other attacks, then murder us immediately because she's unaffected by anything and powerful.

I figure this must be on of the New rituals or something, but I couldn't for the life of me find it. R/smite pls help