Not sure why this happened and have never seen it happen. The text of Professor Onyx's sacrifice ability says "each opponent sacrifices a creature with greatest power amongst creatures they control. I had both Edgar Charmed groom and Henrika domnahi on the field. My opponent used this ability after I think a meathook massacre and BOTH were sacrificed despite this not being the text of Professor onyx. It does not say SACRIFICE all creatures with greatest power they control, it says sacrifice "A" creature.
Not sure why this happened and have never seen it happen. The text of Professor Onyx's sacrifice ability says "each opponent sacrifices a creature with greatest power amongst creatures they control. I had both Edgar Charmed groom and Henrika domnahi on the field. My opponent used this ability after I think a meathook massacre and BOTH were sacrificed despite this not being the text of Professor onyx. It does not say SACRIFICE all creatures with greatest power they control, it says sacrifice "A" creature.