Hallowed Bar

There is no hallowed ore anywhere in the world. You cannot mine this, you cannot craft it, and there is no alternate route to it: every bar comes off a dead mechanical boss. Your supply is therefore a direct function of how many mech kills you have banked, which is why hallowed gear tends to arrive in pieces across several fights instead of all at once.

Selling the spares once you have the gear you wanted is the obvious move and the wrong one. Turning leftovers into Pumpkin Moon Medallions and actually running the event pays out far beyond what the raw metal is worth, before you count everything the event itself drops. It costs an evening rather than a click, which is the only reason not to.

Hallowed armor is the standard answer, and the head slot has a separate variant for each class, so one body and legs covers whichever build you settle into. The bars also feed the Optic Staff and a long list of later additions, so resist dumping the whole stack into armor after the first mech kill.