If I have no units on battlefield and my opponent has 1 unit (Ahri) and 1 Hidden Card, and I use a spell to deal damage to kill Ahri, does the Hidden Card get trashed or can my opponent reveal it as a reaction?

Ruling: The opponent can reveal the Hidden Card as a reaction on the same chain as the kill spell before it resolves, preventing the Hidden Card from being trashed.

Sequence:

• You cast a spell targeting the opponent's only unit on the battlefield
• Before the spell resolves, the opponent can reveal their Hidden Card as a reaction
• If revealed, the Hidden Card enters the battlefield and prevents itself from being trashed
• The spell then resolves

Nuances:

• Hidden Cards cannot be revealed as a reaction on the same turn they are played
• If a player has no units remaining on a battlefield with a Hidden Card, that card gets trashed
• Not all Hidden Cards are units (e.g., Hidden Blade), so targeting the only unit can still force suboptimal plays
• When a Hidden Card is flipped up, that's when it counts as being played and can be responded to (e.g., with Windwall)