Kuroka using Youjutsu
Youjutsu (lit. "Demon Arts") is a technique used in High School DxD.
Youjutsu is the Youkai form of spellcraft and is mainly unique to the Youkai. Currently, Kuroka and Sun Wukong are the only Youkai that have used Youjutsu, while Koneko is still training on how to to properly use it.
Youjutsu is an ability unique only to Youkai and is similar to creating and using illusions.