Snape was bent over his cauldron late into the night, the Eldritch brew boiling and bubbling, wisps of energies swirling in a sinister miasma about the surface, when suddenly the window lit up with a green glow from without.
The potions master looked up, his customary scowl deepening, but in a flash the glass shattered and Snape was hurled backwards against the wall. Suddenly a glowing magical ring descended onto his finger, a green nimbus enveloped him and began to shine from his eyes. Snape began to speak, saying..
"In brightest day, in blackest night,
No evil shall escape my sight
Let those who worship evil's might,
Beware my power... Green Lantern's light!"
His black robes transformed, taking on the symbol of the Green Lantern Corp. Though Snape was brooding, dour and, as any Hogwarts student or faculty could tell you, downright byronian and cantankerous, the ring had sensed the deep courage in his heart, the willingness to sacrifice for the greater good.