From the Kingdom Hearts Wiki: A world of information not accessible by Gummiship
In Kingdom Hearts Union χ, Abyss Assault is a special attack that targets all enemies. It costs 1 special attack gauge to perform. It raises speed-based strength by 1 tier for 1 turn and lowers speed-based defense of all targets by 1 tier for 1 turn.
Learning Abyss Assault
Kingdom Hearts Union χ