The Song Card isn't a dangerous card, but it's powers can confuse people. Song has the ability to copy a beautiful singer, then sing in their voice. Madison fell prey to Song because she herself is claimed to be the best singer at Reidenton Elementary.
In Episode 23, Song appears to copy Madison. I think they competed in a singing contest which Madison won, which also allowed Sakura to seal it. When summoned, it will sing in the voice of whoever it is commanded too. This helped to save Madison from a dangerous ordeal in the Sakura Card series, and helped to later on capture Voice.
