I was just doing the daily campaign and this happened: If you can't tell, two of my champions (the Lonx Hunt Leader and the Lonx Chaneller closest to her) are apparently engaged with an empty square (if you're wondering, that Azaran is definitely mine and wasn't possessed or anything). Next turn, I could move around that square (paying AP to disengage), but wasn't allowed to move into it: When I moved a champion adjacent to that square, I noticed that it said I'd become engaged with a Spirit Singer. However, as you can plainly see, there was no Spirit Singer there. If it was invisible, then my moving into it should have revealed it. If not, then there's no reason why my champions should have ended up engaged with it.