The sphinx grinned down at the three pony sisters, fangs glinting.
"Very well." Its tail lashed. "I shall depart. However—"
The relief on the ponies' faces froze.
"—you must each pay my price." Its gaze bored into the unicorn. "To survive eternity," to the pegasus, "or to die for it."
To the earth pony, the eldest sister, the sphinx said, "Alive or dead?"
"Terra, you can't—" The unicorn began, but Terra silenced her, lest the the wrong thing be said and doom their task.
At last, Terra resolved herself and met the sphinx's fixed stare. "Immortality is a violation against nature. Even had I the fortitude to endure it, I would abhor myself."
The sphinx blinked. Terra crumpled on the spot, as if struck by a stone hammer.
The sphinx grinned again at the unicorn. "Alive or dead?"
The pegasus, trembling with terror, whispered a plea to her sister, "Celestia…"