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."
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."