Mentor stumbled from the stolen vesel. He had failed in guarding the canisters of Energy X, but he might be able to guide those with it. His name was, after all, Mentor.
However, as he'd crashed, one canister hit him full in the face with a dose of it, and he was currently facing the consequences thereof.
At first, he heard a voice saying, "If we work together, no evil in this world could stop us. What do you say?"
Unfortunately for Mentor, his decision had already been made for him.
The first thing that happened was his body colour darkening as lavender fuzz started growing in. He felt new information entering his mind; spells, potions, books, more besides!
Briefly, his hearing was lost as new equine ears grew in, his head shrinking in size to adapt to a new shape.
Much of the muscle he had built over the years began to soften, burning off to allow his continued transformation. His chest in particular expanded by a few sizes.
His feet were replaced with hooves, his eyes widened (quite literally) and then shifted to a darker violet in hue.
Not having hair ever before, he was surprised when a rich violet mane and tail started growing in.
A long horn speared from his newly reformed skull, shooting sparks as it connected to his system.
Speaking of, a new organ started to form. One filling with energy as it grew. Though smaller than most organs, each little bit of space counted an exponential increase in energy!
Next, a core of solid violet formed within him. It felt of strong magic being worked and the satisfaction of hard studying paying off. Just after came one of a calm blue. It felt of bonds forged with others through adversity and good times with friends. Of undying loyalty to many people.
More internal changes occurred, too numerous to mention, but finally the glow of the tainted Energy X left Mentor's changed form, emblazoning her thighs with a purple starburst surrounded by tens of little lights.
With but a thought, a metallic shamanic staff came to hand. Unnecessary, really, but it would serve.
"Mentor may be dead," she said aloud, "but his mission did not die with him. I will continue what 'he' started. From this day forth, I am Twilight Sparkle: Archmage!"