I had something pretty weird happen today while killing kings.
I had attacked one just east of x-roads on merchant street, after a while another one comes and I think i killed one, when all of a sudden the other one goes invisible. So I move a little and he becomes visible again and eventually a 2nd one comes. Now I am turning invisible, but still hitting them, killing them both and becoming visible again in the process. The ones I had killed almost immediately respawned and in the end I had killed each King twice (but only one set of graves appeared).
Really strange things...