If you haven't already, read the prequel: Part 1
Gray was not the easiest pupil Ignis had ever taught. In fact, he was so active, it took several months before he could meditate more than 10 minutes. Ignis stood by patiently, ever watchful, noting as his pupil sharpened his mind and grew stronger.
One night came, dark as the darkest pitch, when the air was cold. Colder than normal, so cold that even the machines that controlled the climate could not change it. Gray shivered and asked Ignis, "What's going on? Is the control center failing?"
Ignis shook his head gravely. "No. A monster awakens, hungering for your newfound power. You must fight it, and defeat it. It is a... test."
Gray started. "You mean you summoned it here?"
Ignis replied, rather harshly, "No! I would not summon something so evil. It always appears, has always appeared. It is older than the machines that now control us." He swept back into his cave and came back out wielding a long, orange-red trident, radiating heat that drove the otherworldly chill from the air. "This," the Diabolist explained, "is an Inferno Fork. They were made by my people, the Diabolists, long ago. But not a one has been created in an age, and my race's power fails without them."
Gray took the rod, pulling on black gloves to shield his hands from the heat. He examined it, then closed his eyes. A ball of flame shot from each of the trident tips, pulling into one larger one that shot forward, detonating in a massive explosion against the opposite wall. The metal covering that was starting to seep into the Corruption retreated, ever so slightly.
Ignis nodded approvingly. "You are taking a start into real magic," he said quietly. "Now go out, stand at the top of the chasm. Wait for your challenge."
Gray stood, silent and still, waiting for his enemy.
It was past midnight when he heard something, and then saw something. A roaring, grinding, crunching noise grew louder and louder, seemingly coming from all around. Spotlights, red and brilliant, soared in sweeping arcs over the ground.
It came. A massive, metallic skull-and-crossbones creature with lights flashing over its body. An arm glowed red here, and launched a barrage of lasers. A cannon exploded with a puff of smoke, sending a ball almost as large as Gray downwards. A vice made a grab for the warrior, and narrowly missed. Sawblades sliced left and right, up and down.
Gray appeared undaunted, but gritted his teeth, a sure sign he was under pressure. Swinging his Slime Staff, he summoned his slime, which darted around the skeleton's arm bones, keeping it distracted. Gray drew his Inferno Fork, launching fireballs all the while. His True Night's Edge seemingly slid out of his hands, so fluid was its drawing. The skeleton recoiled at the evil aura emanating from the blade, chilling the air, but it soon returned, its saw blade making a sweep directly at Gray.
The warrior parried, all the strength in his wiry limbs pouring into keeping his sword grounded against the massive force. He had forgotten the vice.
It swept behind him, grabbing him and bringing him up to eye level with the massive skull. Bolts ground and crunched as the jaw of the mechanical monster opened and closed, revealing void behind its shining teeth.
Gray's arms were pinned to his side by the vice's crushing grip, the saw blade below it, ready to slice him in half. The eyes of the mechanical monster glowed red, heating up for a laser blast beyond all others.
Gray closed his eyes and gritted his teeth. He struggled to think about nothing but heat, the heat with which he had trained. The heat in the Inferno Fork, strapped to his back. In the arms of one who loved.
Skeletron Prime was shredded by a furnace blast extending its tendrils of pure heat throughout the sky. Gray tumbled out of the sky, unnoticed, while an explosion lit up the skies of Terraria. The monsters paused in their patrols of their land, absorbed the sight with heat sensors, and rumbled toward the Corruption.