Is the stun from Zenith Blade a delayed trigger? Does it apply the legends buff before the showdown begins?

No, the stun from Zenith Blade is not a delayed trigger, and it does not apply legend buffs before a showdown begins (if the spell is played during a showdown).

Here is the breakdown based on the rules and retrieved information:

1. Is the stun a delayed trigger?

No. According to the card text, Zenith Blade is a spell with an [Action] ability that performs an effect immediately upon resolution.

• Rule 390.2 defines a "Delayed Trigger" as a triggered ability that identifies a specific time or condition in the future.
• Zenith Blade simply says "Stun an enemy unit." This is an immediate game action executed when the spell resolves, not a delayed ability that waits for a specific future trigger condition.

2. Does it apply legend buffs before the showdown begins?

No.

• If you play Zenith Blade during a showdown: The showdown has already begun. Zenith Blade resolves as an item on the chain, and if the stun triggers a legend ability (such as Leona's, assuming you are using a card that interacts with stunning), that trigger is added to the chain and resolves during the showdown, not "before it begins."
• If you play Zenith Blade outside of a showdown: The spell resolves, stuns the target, and triggers any relevant abilities. If this results in a unit moving into a battlefield with enemy units, that move may initiate a showdown, but the stun effect itself is an immediate interaction that occurs before the new showdown is staged.

Important Nuances:

• Timing: Zenith Blade requires you to target a unit to stun it. As confirmed by RiftJudge FAQ #5626 and #2180, you can target an already stunned unit with Zenith Blade to gain the move effect, but the stun portion will do nothing, and "When you stun" triggered abilities will not trigger.
• Legend Buffs: If you are referring to a legend ability that triggers "When you stun," that ability is placed on the chain as a triggered ability. Per the showdown timing flow, triggers added during a showdown resolve before combat damage is dealt, but they do not happen "before the showdown begins"—they are part of the active showdown's chain.

Disclaimer: This is my interpretation based on the card text and core mechanics. No official FAQ exists for the specific phrasing "before the showdown begins" regarding Zenith Blade's stun timing.