I dont want to complain really, but after this KINO level of a dramatic showdown this feels a little too much like an asspull - yes the witch and Piwi have technically been introduced way earlier, but nothing indicated Piwi had this kind of power, it was heavily implied it was the song which had the effect on the dark warrior before, not Piwi..
To begin with, did they ever explain what Yggdrasil even is in this setting?