If playing Sett Brawler with Volibear legend, can both 'When X is played' effects trigger, with Volibear's effect resolving first to make Sett mighty, allowing the rune draw?

Ruling: No, only Sett's trigger will activate. Volibear's trigger condition requires the unit to be mighty when played, and Sett is not mighty when played, so Volibear's effect does not trigger at all.

Nuances:

• The trigger condition "When you play a mighty unit" means the unit must already be mighty at the moment of play for Volibear to trigger
• If Volibear said "When you play a unit, if it was mighty..." it would work differently, but that's not how it's worded
• Passive Abilities take effect just before a unit officially enters the field, which can sometimes allow non-mighty units to be played mighty (e.g., Origins Draven at 2 points, or Qiyana at Trifarian Warcamp)