At least one Modifier will always apply when spending Threads. Spending more Threads improves the chances of multiple Modifiers applying.
Bestiary Modifiers which can be applied by Threads:

Area contains additional Red Beasts.

Area is invaded a large number of Red Beasts.

Area contains the Black Mórrigan.

Area contains additional Sacred Fauna Red Beasts.

Yellow and Red Beasts are empowered by Wildwood Wisps of all types.

Enemies near Einhar explode on death, dealing 25% of their maximum Life as Physical Damage.

Yellow Beasts in area are Nameless Beasts.