Rats on maze level?

Hey all, was doing a Soul run on hard, just trying it out for the first time. Anyway, I was doing well until I went to maze and fought some enemies. Suddenly the entire floor was covered in what looked like a swarm of rats going on endlessly from on side of the map to the other.

My question is twofold:

1. was this a spell cast by an enemy? Or was this a map specific instance?

2. has anyone else experienced this? If so how did you counter it if you did? I’ve played this game several times through and literally never experienced this haha.

Thanks all!