When can my opponent play Flash in response to Brynhir Thundersong being played?

Ruling: Your opponent can play Flash after all other triggered abilities have resolved, while Brynhir's "When you play me" lockdown trigger is still on the chain. Brynhir enters play immediately as a unit (it does not linger on the chain), but its lockdown trigger goes on the chain as a separate Pending Item that can be responded to.

Sequence:

• Brynhir is played and enters play immediately upon finalization
• Brynhir's "When you play me" trigger goes on the chain
• If opponent responds with Tideturner from Hidden, it also enters play immediately
• Tideturner's "When you play me" trigger goes on the chain above Brynhir's trigger
• Tideturner's trigger resolves, swapping with Hwei
• Hwei's "When I move" trigger goes on the chain and resolves
• Chain now has only Brynhir's trigger remaining; priority passes and opponent can play Flash as a reaction
• Flash resolves, then Brynhir's lockdown trigger resolves

Nuances:

• Units enter play immediately upon finalization and do not linger on the chain like spells do
• The lockdown effect only applies after Brynhir's "When you play me" trigger resolves, not when Brynhir enters play