Diana, Lunari

Diana's ability resolves last in the initial chain, after all attack and defend triggers.

When combat opens with a combat showdown, the attacker and defender designations are assigned to units.^[459.2.b] 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.^{[459.2.d][459.2.d.1]}

Diana's "when a showdown begins here" trigger is placed on the initial chain as the very first item, before any of those attack and defend triggers. Because the chain resolves newest-item-first, Diana's ability resolves last — after every attack and defend trigger from that same initial chain has already resolved.

Diana's ability — "When a showdown begins here, you may pay 1. If you do, Predict, then reveal the top card of your Main Deck. If it's a spell, draw it." — breaks into three distinct parts.

"When a showdown begins here" is the trigger condition. It fires whenever a showdown begins at Diana's battlefield, whether that is a combat showdown or a non-combat showdown.

"you may" appears as the first part of the effect, so the controller chooses whether to place the triggered ability on the chain at all when the condition is met.^[383.3.a] If you decline, the ability never goes on the chain and nothing happens.

"pay 1. If you do, …" is resolution-time effect text. "Pay 1" is not a cost within instructions — those are identified by the "[do X] to [do Y]" template — so the ability has no cost at finalization.^[742.1] Everything here executes when the ability resolves. What happens with the payment depends on the state of your rune pool:

The "If you do" conditional — Predict, then reveal the top card of your main deck — only applies if the Energy was paid, whether consumed from floating resources or generated via Add reactions. If no Energy was paid, the remainder of the ability is skipped entirely.