Diana on Ravenbloom Conservatory. What triggers first, the when I defend or the when a showdown begins?

Diana's "When a showdown begins here" triggers first. Ravenbloom Conservatory's "When you defend here" is a Defend Trigger that only fires once the Defender designation is assigned — and that happens after the showdown begins.

Order of events when the opponent moves in and Combat opens (Rule 464.2):

1. Showdown begins — Task 1 "Start of combat or showdown effects happen now" → Diana triggers (Rule 464.2.b).
2. Task 2 establishes Attacker/Defender → you gain the Defender designation → Ravenbloom triggers (Rules 464.2.c, 383.4.f).

One practical nuance: both go on the Combat Chain in Task 4, with the Defender's triggers placed last (Rule 464.2.e.1). Since Diana's became pending first, Ravenbloom's sits on top of it — so on resolution (LIFO), Ravenbloom's reveal resolves before Diana's Predict/draw. And if the Showdown were non-Combat (no enemy unit present), you'd never get the defend trigger at all (Rule 383.4.f).