Astaroth (SCIV) R destroy 1 foundation: After your opponent plays an ability that modifies a control check, cancel that effect.

Communing with the Ancients F commit: Your control checks to play attacks for the rest of the turn get +1.

Can Astaroth use his reaction against this ability, since when CwtA does not actually give a bonus until an attack is played.

If yes, does he react as soon as the ability is played, or as soon as i go to make a control check for an attack?

Yes you can. React after the ability is played. then there are no more +1 checks for attacks for that turn

The response says it responds when an ability is played so that's when it responds. Doesn't matter that the check modification isn't immediate.