for nasus. do you tap the legend as soon as you pay? or once it resolves. aka if someone reacts to me empowering nasus. do i get my +2 runes first or after it resolves?

You tap Nasus immediately when the trigger is finalized on the Chain — before the ability resolves — but the +2 runes only happen when it resolves.

Curator of the Sands's "exhaust me to" is a cost built into the trigger, paid during finalization when the ability goes onto the Chain (not at resolution). So the moment your 7+ cost card is played and the trigger finalizes, Nasus is exhausted — that's the tap, and your opponent can't stop it because finalization happens without passing priority.

Then the "ready up to 2 runes" is the effect that resolves last on the Chain. If your opponent reacts to the ability (e.g., kills Nasus) before it resolves, you still get the runes — once a triggered ability is on the Chain it resolves regardless of what happens to its source (FAQ #5169, #6130).

One catch: the reverse isn't true — the runes are not ready at the moment you tap. If you're tapping Nasus to fund something that happens on the same trigger window, the runes arrive only when the ability resolves.