He'd have to know what I'm doing. It isn't an attack, like I said... it's subtle, he'd have to notice the fire slowly growing to realize his energy is being drained out of him.

The rate can be controlled, if it's done slowly, he won't notice until he's exhausted, but if it's done quickly, it'll feel like crushing. Like your life force is being rapidly drained out of you, mostly because, well, that's exactly what's happening.