Can an opponent use a reaction to reduce a unit's might below 5 to prevent Volibear Relentless Storm's legend ability from triggering and channeling a rune?

Ruling: No, you cannot prevent Volibear's trigger by reducing a unit's might after it is played. When a mighty unit (5+) is played and enters the board, Volibear's triggered ability is added to the chain and will resolve even if the unit's might is later reduced below 5.

Sequence:

• A mighty unit is played and enters the board
• Any "Play" triggers (including Volibear's) enter the chain
• Players receive priority and can respond
• When Volibear's trigger resolves, you tap him and channel a rune regardless of the unit's current might

Nuances:

• Triggered abilities are checked only once (when they trigger), unlike targets which are checked twice (when added to chain and when resolving)
• This differs from Fox Fire, where changing a unit's might can invalidate the target because target legality is rechecked on resolution