Thereupon to protect him the excellent and flaming discus Sudarshana was despatched by the Great God. Thereupon thousands of Samvara's illusions were baffled by the quick-coursing discus for the protection of that boy.

In plain words: Vishnu sends his blazing discus, Sudarshana, to protect Prahlada, and it destroys all of Samvara's illusions.