Powerful shrine maiden defeats the chief of oni but her younger sister gets into a trap set up by a small oni witch and as a consequence she loses. Later older sis is fused with the oni she killed and transformed into oni herself.

Older version of this work was posted here:
https://e-hentai.org/g/2024165/6dbc6e47ac/

Difference is that day after it was posted author decided to update his work and make the shrine maiden more muscular as an oni, since it fit better and text was describing such change even though nothing like that was depicted in illustrations. This is that newer version. Also added textless version and version with sound effects only.
1 - 135 text version
136 - 270 version with sound effect only
271 - 405 textless version

If you liked this work you can support the author and buy it here:
https://www.dlsite.com/maniax/work/=/product_id/RJ288835.html

Tag 'horns' is inapplicable to oni.

https://ehwiki.org/wiki/oni

Base +28, Constipated_Guard +6, Lumberjac108 +7

Interesting...

Base +8