Diana, Lunari

Diana's ability resolves after all attack and defend triggers generated as that combat opens.

When combat opens with a combat showdown, the attacker and defender designations are assigned to units.^[464.2.c.3] Attack triggers fire when a unit gains the attacker designation, and defend triggers fire when a unit gains the defender designation.^{[383.4.e][383.4.f]} Those triggers are then added to the combat chain, with the attacking player placing theirs first, followed by others in turn order, and the defending player last.^{[464.2.e][464.2.e.1]}

Diana's "when a showdown begins here" trigger is a start-of-showdown effect, which the combat-opening sequence processes as its very first step — before designations are established and their attack and defend triggers are added to the chain.^[464.2.b] Because the chain resolves newest-item-first, Diana's ability resolves last — after every attack and defend trigger from that same combat chain has already resolved.^[340.1]

You pay it while Diana's triggered ability is being finalized, before opponents can react.

Because "you may" is the first part of the effect, you choose whether to perform the ability during finalization.^[383.3.a] The immediately following "pay 1 to Predict" is a cost within instructions, so the payment is treated as the triggered ability's base cost and must be paid for the ability to finalize.^{[383.3.b][383.3.b.1]} You may activate Add reactions while paying this cost.^[444.2.c]

If you do not pay, the ability does not finalize and none of its effects occur. If you pay, the ability is placed on the chain; when it resolves, you Predict, reveal the top card of your main deck, and draw that card if it is a spell.