超能力 = super power, ability
正体 = true form
そのままの翻訳です!
彼の超能力の正体は、実は赤外線だったのだ
The true form of his ability was actually infrared radiation
例:
彼の超能力は風を操る力じゃなくて、その正体、実はあらゆるベクトルを任意に操作する能力だった。
His power wasn't to control the wind. The true form of his power was actually to freely control all vectors.