Diana, Lunari

When combat opens with a new showdown, Diana's ability triggers first but resolves last, after all attack and defend abilities generated immediately afterward.

Start-of-showdown effects happen before units gain their attacker and defender designations, so Diana triggers before those designations trigger attack and defend abilities.^{383.4.e383.4.f466.2.a466.2.b466.2.c.3} These are consecutive combat-opening tasks with no priority between them, so all resulting abilities are pending on the combat chain before any is finalized or can resolve.^{334.1334.2466.2.d} The newest chain items resolve first, so the attack and defend abilities resolve before Diana's ability.^340.1

If combat opens during an existing showdown, the showdown does not begin again, so Diana's "when a showdown begins here" ability does not trigger.^466.1466.2

You pay it while Diana's triggered ability is being finalized, before opponents receive the normal opportunity to play Reactions to the ability.^407.4

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 becomes the triggered ability's base cost and must be paid for the ability to finalize.^{383.3.b383.3.b.1740.4.a} While paying, you may activate an eligible [Add] ability with [Reaction] to provide the energy.^445.2.c

If you decline to pay, the pending ability is removed from the chain without finalizing.^405.2 If you pay, it becomes a finalized chain item, and opponents may react before it resolves.^407.2407.4 When it resolves, you [Predict], reveal the top card of your main deck, and draw that card if it is a spell.