Rat covenant is **not** a cheap way to invade, like at all, I'm learning. You pull people to you (most of them 'exterminators') and if you lose, you're out an Effigy (which you don't get back by winning). 

Still a blast, as I've had some really awesome fights, but there's essentially no pressure on your victims, and the rat keeps all the risk (though a lockstone **is** a pretty sweet reward)). You know what it's like? It's a Canadian Cat Covenant. 'Sorry we had to fight, eh, hope I didn't bother you.'