Invisible Enemy in Anchordeep- Was it a bug or am I stupid?

I had an incredible run going, had like 6.5 hearts and was only a couple doors away from one of the mini-bosses of Anchordeep, and then a room wouldn't unlock after I had killed everything on the screen. I took damage from something invisible, then noticed their....I don't know, footprints? Something burrowed? I can't say for sure what exactly it was, but I couldn't damage it with my curse or with my weapon. Is this an enemy I simply didn't know about (it was only my 2nd Anchordeep run) or a bug? If it's the former, how do you kill it?