My thought would be "metal-eating bacteria" - essentially a Plague of Rust. Depending on what it produces as a byproduct it could still be nasty to biologicals, too.
It could be the deadliest substance in the Universe.
Dihydrogen monoxide.
Will not only destroy Yor but most of the biological life as well.
The Yor because it dissolves metal.
Most biologicals because if they breathe it, they will die.
