Arrow Volley Trap
Instant — Trap
If four or more creatures are attacking, you may pay 
rather than pay this spell's mana cost.
Arrow Volley Trap deals 5 damage divided as you choose among any number of target attacking creatures.
Steve Argyle
Zendikar (ZEN)
• #2Rulings
You divide the damage as you cast Arrow Volley Trap, not as it resolves. Each target must be assigned at least 1 damage.
(wotc • 2009-10-01)
Casting a Trap by paying its alternative cost doesn’t change its mana cost or mana value. The only difference is the cost you actually pay.
(wotc • 2009-10-01)
Effects that increase or reduce the cost to cast a Trap will apply to whichever cost you chose to pay.
(wotc • 2009-10-01)
You may ignore a Trap’s alternative cost condition and simply cast it for its normal mana cost. This is true even if its alternative cost condition has been met.
(wotc • 2009-10-01)
Rulings
You divide the damage as you cast Arrow Volley Trap, not as it resolves. Each target must be assigned at least 1 damage.
(wotc • 2009-10-01)
Casting a Trap by paying its alternative cost doesn’t change its mana cost or mana value. The only difference is the cost you actually pay.
(wotc • 2009-10-01)
Effects that increase or reduce the cost to cast a Trap will apply to whichever cost you chose to pay.
(wotc • 2009-10-01)
You may ignore a Trap’s alternative cost condition and simply cast it for its normal mana cost. This is true even if its alternative cost condition has been met.
(wotc • 2009-10-01)