Complex sentence: Matamune gave his power to Yoh, who was then able to defeat the Oni which was terrorising Anna, although it uses up his remaining furyoku and meant that Matamune was unable to return to his normal form.
Split into simpler sentences: Matamune gave his power to Yoh, who was then able to defeat the Oni which was terrorising Anna. However, it used up his remaining furyoku and meant that Matamune was unable to return to his normal form.