Does blue diana ability go first or defend and attack triggers go first?

Diana, Lunari's "when a showdown begins here" trigger goes on the chain first, but it resolves last — after both the attack and defend triggers.

Here's the timing: when combat opens, start-of-showdown effects (like Diana's trigger) happen before the attacker/defender designations are established (Rule 464.2.b–c). So Diana's ability becomes pending first and sits at the bottom of the chain. Attack triggers are added next, and defend triggers are added last. Since the chain resolves last-in-first-out, the defend triggers resolve first, then the attack triggers, then Diana's ability resolves last.

Concrete example (RiftJudge FAQ #10544): if Kha'Zix, Evolving Hunter attacks into Diana, his "when I attack" trigger resolves before Diana's trigger — so he can kill Diana before she gets to Predict and draw.