If you look, the tree above Youmu has its branch cut and her sword is embedded in its trunk. So, Reisen hypnotized Youmu into thinking the tree was her real body.
If you look, the tree above Youmu has its branch cut and her sword is embedded in its trunk. So, Reisen hypnotized Youmu into thinking the tree was her real body.
That would be the logical assumption here. No magical regeneration or anything going on, just Reisen using her abilities with razor sharp precision.