So, what's this hydrofluoric acid thingy?

A strong poison that binds to calcium.

If you get any on your body, it will seep into your bones and cause an agonizing death by eating you from the inside out.

But since you're immortal, Princess,

you'll only writhe in pain for a little while as your bones get corroded, I think!

'Shun goku satsu' style.