Instrumentality was unbreakable, it seemed clear he was referring to the noose.
What?
I'm sorry but what? a god know calls a length of rope "instrumentality" must be pretty damn dumb.
I have no idea how you get that from the context of the episode.
"No idea"? Please don't take this as an insult, but that seemed obvious. Trelane declared his instrumentality unbreakable, then the shadow of the noose was cast on the wall. The connection eluded you?
Do you know what "instrumentality" means? In FORBIDDEN PLANET the captain and doctor talk of "a civilization without instrumentalities." They didn't mean a civilization devoid of dials and gauges. They meant no physical tools—including rope. Or at least the Krell had the ability to "will" a length of rope into being—which puts Trelane in the same category. Both still used machines, or at least one big machine, but smaller manual or powered tools as we know them no longer existed.