Talk:Echidna/@comment-112.198.169.196-20200903051057/@comment-99.232.106.176-20200917201700

100001% not from Satella/The Witch of Envy, it has been established Echidna detests her but did not instigate conflict out of fear the other witches might take sides in the matter.

As stated by @BlackReaperSasaki no official answer has been declared.

Best we can do is theorize, for all we know the failed soul transfer could be the cause (hence Ryuuzu's body still stored in crystal) or something as stupid as she fell down a flight of steps... lol

Joking aside, I'd suspect it was her failed attempts to transfer herself that resulted in her untimely death. From what we see of her spirit she still looks young so there's a good chance the form we see her in is the final form she had before her death.