Some hybrid peonies are more susceptible to rot than others. Your crown of your ES is showing a viable eye and this is good. If you are not a pot person like me (I killed more potted plants than I can remember), you can cut off the rotted parts, dust it with rooting hormone, and plant it in a well drained hole with this mix: 40%native soil, 30% compost, 30% sand. Plant the root at the surface of the ground or the eye at ground level. Then mound it with the mix. There will be a small mound of soil which may look weird but over time it will compact and sink. What you want to prevent is for the root to sink too far down. This coming winter, make sure you mulch it well so that it will not go through thaw and freeze cycles before growth. Good luck!
