The Twilight Limit

Named after Princess Twilight, the Twilight Limit is the maximum amount of magic a unicorn can safely contain. If a unicorn passes the Twilight Limit, its hypothesized that the unicorn would have no control of the magic, which would quickly consume the unicorn, making them a pure manifestation of magic.

Applejack: "So what would happen if somepony past the Twilight Limit y'all are talking about?"

Twilight: "We don't know actually, but we theorize that they would eventually be consumed by the magic and become pure magical energy. Who knows what could happen after that."