If both players pass priority with nothing on the stack, does the phase end immediately?

When both players pass priority consecutively with an empty stack, the current step or phase ends. This is governed by CR 116.3d: if all players pass in succession and the stack is empty, the phase or step ends. The game then advances to the next step or phase in sequence.

It is important to note that the game moves to the next step if the current phase has multiple steps (e.g., the Combat Phase has Beginning of Combat, Declare Attackers, Declare Blockers, Combat Damage, and End of Combat steps). If there are no more steps in the current phase, the phase itself ends and the next phase begins.

Some steps and phases are skipped automatically if certain conditions aren't met — for example, the Declare Blockers Step is still entered even with no attackers, but the Declare Attackers Step can effectively be passed through quickly. The rule applies uniformly: double-pass on an empty stack always closes the current step or phase.

Concrete example: It's your Main Phase 1. You cast a Shock, it resolves, and now the stack is empty. Both you and your opponent pass priority in succession. Because the stack is empty and both players passed, your Pre-Combat Main Phase ends and the game moves to the Beginning of Combat Step.

Reference rules: CR 116.3d covers the condition of all players passing with an empty stack, and CR 500.2 explains how phases and steps end and transition.