Does the effect of Reaver's Row trigger at the start of a showdown or the end of a showdown?

The effect of Reaver's Row triggers at the start of a showdown, not the end.

According to RiftJudge FAQs (such as #1599, #1984, and #4884), Reaver's Row functions as a "when I defend" triggered ability that is placed on the initial chain created when a showdown begins.

Key Timing Details:

1. Triggering: The ability triggers at the start of the showdown when you become the defender.
2. Targeting: You must declare the target for the ability when it is placed on the initial chain.
3. Resolution: The decision of whether or not to actually move the unit (the "may" part of the ability) is made when the ability resolves.
4. Interaction: Because it is on the initial chain, players can respond to this trigger with reactions, but they cannot play action speed spells until the entire initial chain has finished resolving.

Sequence Summary:

• A showdown begins (initial chain is created).
• Reaver's Row triggers and is placed on the chain; you declare the target unit.
• Players may add reactions to the chain.
• The chain resolves (LIFO - Last In, First Out).
• When the Reaver's Row ability resolves, you choose whether to move the targeted unit to your base.