Credit goes to https://www.furaffinity.net/user/srbunny649

Note: He's flailing his arms due to having bad balance.

Magus and Ember of the umbran clan are known demon summoners through various dark arts, dancing being the umbran main way to do it. Spectra made sure to train both Magus and Ember in dancing for summonings. She also wanted Magus to have a backup way since he more of using summon circles.
Part of the training was dancing and keeping balance in thick diapers, both he and ember always wore thick diapers to help them improve dancing.

Ember who had a difficult time and Magus barely knew any dance steps at all. Sometimes she gave them private dance lessons and Magus really needed the most given he didn't know the basics of dancing. He can barely keep his balance and often struggled just to stand. Spectra wants him to atleast master thick balance.
Since Magus struggled Spectra kept trying to improve him despite him not wanting too since he can't dance at all, what made him keep trying is Spectra promised to take him to underworld library if he did good.

Even as he became a Delphox he still can't dance, but he did eventually improve in balancing and fighting in thick diapers.

End.

Magus belongs to Me
Braxien belongs to Game Freak