Ancient Stone Idol
Artifact Creature — Golem
Flash
This spell costs 
less to cast for each attacking creature.
Trample
When this creature dies, create a 6/12 colorless Construct artifact creature token with trample.
Josh Hass
Rulings
If there are enough attacking creatures, Ancient Stone Idol's cost can be reduced to {0}.
(wotc • 2018-07-13)
The total cost to cast a spell is locked in before you pay that cost. For example, if you control five attacking creatures, including one you can sacrifice to add {C}, Ancient Stone Idol costs {5} to cast. Then you can sacrifice the creature when you activate mana abilities just before paying the cost, and it still costs only {5} to cast.
(wotc • 2018-07-13)
Ancient Stone Idol's cost is reduced for each attacking creature, not just creatures attacking you. It even counts your attacking creatures.
(wotc • 2018-07-13)
Printings
Rulings
If there are enough attacking creatures, Ancient Stone Idol's cost can be reduced to {0}.
(wotc • 2018-07-13)
The total cost to cast a spell is locked in before you pay that cost. For example, if you control five attacking creatures, including one you can sacrifice to add {C}, Ancient Stone Idol costs {5} to cast. Then you can sacrifice the creature when you activate mana abilities just before paying the cost, and it still costs only {5} to cast.
(wotc • 2018-07-13)
Ancient Stone Idol's cost is reduced for each attacking creature, not just creatures attacking you. It even counts your attacking creatures.
(wotc • 2018-07-13)