I Really Don’t Want To Learn Forbidden Spells - Chapter 1.5

So does anyone know what spell did he used? He just literally raised his arm.

Skill 1 is Cast a Forbidden Skill once a week.

Skill2 is Immunity to Dark spell backlash.

Skill3 is Shield.

if that was skill 1 then he’d almost be powerless for a week? he can only shield?