The Books of truth timing

[Krol]

At the end of the Modify dice step of an attack from Magneto, my opponent uses Recalibration matrix. After the effect can I play The Books of Truth or has my opportunity passed? 

[Negoldar]

On 12/11/2023 at 8:27 AM, Krol said:

At the end of the Modify dice step of an attack from Magneto, my opponent uses Recalibration matrix. After the effect can I play The Books of Truth or has my opportunity passed? 

When both players have an effect that resolves in a given timing step of an attack, the attacker resolves first, then the defender. 

In your example, Magneto’s player must play their rerolling effect before the defender.