Rule 344

A Showdown begins when Control of a Battlefield is Contested during a Cleanup and the turn is in a Neutral Open State.

• 344.1 If Control of a Battlefield is Contested between two players, then a Showdown will be opened as the first step of…
• 344.2 If Control of a Battlefield is Contested, there aren’t units controlled by different players there, and the turn is in…

Official Core Rules