2024-09-20
Creatures that enter attacking were never declared as attackers, so Shriekwood Devourer's last ability won't take them into account.
2024-09-20
The value of X is calculated only once, as Shriekwood Devourer's last ability resolves. This means that the value of X may be different than you had hoped when attackers were declared if attacking creatures have left the battlefield or their power has changed.