Does Windwall counter the entire Icathian Rain spell or only one instance of its damage triggers?

Ruling: Windwall can only counter the entire Icathian Rain spell before it resolves. Once Icathian Rain begins resolving and its individual instances go on the chain, Windwall cannot counter the original spell anymore, only individual instances.

Sequence:

• Opponent plays Icathian Rain
• You can play Windwall to counter the entire spell before it resolves
• Once Rain resolves, its 6 instances go on the chain individually
• At this point, Windwall can only counter one individual instance, not the original spell

Nuances:

• The timing window matters: Windwall must be played before Icathian Rain resolves to counter the entire spell
• After resolution begins and targeting starts, you cannot counter the original spell with Windwall anymore