Does Sett trigger Volibear's ability when played, since Sett gains +1/+1 on play and could become Mighty?

Ruling: Sett does not trigger Volibear's ability when played. Volibear's trigger condition requires the unit to be Mighty at the time it is played, and Sett only becomes Mighty after his on-play effect resolves.

Sequence:

• When Sett is played, triggers are checked
• At this moment, Sett is not yet Mighty (still 4/4)
• Volibear's condition "When you play a Mighty unit" is not met, so it does not trigger
• Sett's on-play buff then resolves, making him Mighty
• This is too late for Volibear to trigger

Nuances:

• If an effect applied instantly (like Trifarian War Camp), Volibear would see the increased might and trigger
• If Volibear's text read "When you play a Unit, channel a Rune if it is Mighty" instead, then it would work with Sett
• The key difference is whether "Mighty" is part of the trigger condition or part of the effect resolution