Dumbledore sagged against him and Harry cursed. “C’mon, professor, we’re almost done,” he soothed over the Headmaster’s incoherent whimpers.
“Water,” Dumbledore said suddenly.
Can’t hurt. Maneuvering his wand around Dumbledore’s shoulders, Harry pointed at the discarded goblet. Auguamenti, he thought. A thin stream of cold water poured out of his wand and into the goblet, where it promptly vanished.
“Oh, you must be joking.” Harry banished the immediate panic and scowled. Of course Voldemort wouldn’t make it easy. A fluid twirl of his wand conjured a simple cup atop the basin. Auguamenti, he tried again.
The water stayed put this time, and he smiled triumphantly. “Here you go, sir. Drink up.” He gently tilted Dumbledore’s head back and tipped the fresh water into his mouth. The Headmaster swallowed it greedily, more life in him than in the past three minutes since he’d started taking the potion. Harry’s swell of pride disappeared as Dumbledore suddenly began to thrash violently in his grasp. His mouth opened in a silent scream as he bucked out of Harry’s grip, and he fell to the ground with a thump.
“Shit! Stupefy!” The Headmaster stopped his thrashing immediately, and Harry wracked his brain. He hadn’t recognized the potion or its effects and, regardless of his work over the year, there was quite a bit he was still unfamiliar with. Snape had certainly alluded to brews that could have effects the likes of which Dumbledore suffered from, but the lessons had never delved beyond the mention. “Right. Back to Hogwarts,” he told himself. He cleared his mind and dropped a stasis spell on Dumbledore. He was still suffering the mal-effects of the potion even while unconscious; slowing his metabolic processes would hopefully make Snape and Pomfrey’s job easier. Mobilicorpus.
The sound of water being displaced rang loud and clear in the otherwise silent cave, and Harry froze as he made for the boat. He turned and his eyes widened.
The inferi had awoken. Half a dozen bodies had shambled out of the depths behind him and were stumbling toward them. A moment of unadulterated panic seized him and his wand snapped up. The Battering Ram of Rome leapt from the tip a brilliant white and barreled into the first inferius. The bludgeoning spell tore through the corpse’s chest and sent it hurtling into the water two dozen feet away.
Fire, you idiot! Harry screamed at himself as two inferi replaced the one he’d obliterated. Incendios grata! he incanted silently.
A blast of rippling, red flame exploded outward. It caught the first inferius full-on and kept plowing forward. It stymied one, two, three corpses until it suddenly flickered into nothingness. A wave of bone-chilling cold swept through Harry and he staggered back a step.
“No no no no no,” he muttered.
It must’ve been an enchantment, like on the goblet. For the first time, beyond his fear, Harry truly understood Dumbledore’s consistent respect for Voldemort’s ingenuity.
A panicked glance showed more inferi were rising from the depth and making for their location. Thinking quickly, Harry banished Dumbledore into the boat that had carried them to the island. The Headmaster landed safely and Harry wracked his brain for a solution as he cast a wide banishing spell that succeeded in beating back the corpses for an extra second.
The incantation for fiendfyre flashed through Harry’s mind as he backed up toward the boat, but he hesitated. There was too much for him to focus on to have any confidence in controlling the cursed flame.
Inspiration striking him, Harry hastily cleared his mind and centered himself. He inhaled deeply, held it, and focused. His mind centered on the familiar sound of Fawkes’ song, the strength it always gave him, the warmth it sparked in his chest without fail.
“Ignis aeternum!” he roared after a moment of stillness. A great gout of golden flame erupted from his wand and fanned outward. Searing heat beat back the cold in an instant as the phoenix fire consumed the remaining inferi Harry could see.
He whipped his wand in a circle about his head and shoulders, and his fire moved with his will. It swirled in a wide arc around the island, incinerating each inferius the moment it crawled its way out of the depths.
Harry backed up, directing the golden firestorm with precise waves of his wand. His foot met the wood of the boat and he stepped in gratefully. Hunkering down, Harry laid a hand on Dumbledore to keep him steady as his will maneuvered the phoenix fire in a massive circle around the island. It encircled the boat and, with a burst of insight, Harry sharply brought his wand down and pushed.
The fire dropped and the cave was illuminated in brilliant red and gold as the flames flowed out in a massive wave. “Depulso!” Harry said, wand pointed at the island, and the boat lurched forward in a rapid skip across the water.
They followed the light of his fire as it crashed onward, immolating any rising hands that popped up to halt them. Harry let his focus slip as they neared the shore, and the flame winked out of existence. The bite of the cold wasn’t quite as deep as he levitated Dumbledore from the boat and hurried to the exit.
He stepped through the mouth of the cave with relief and tasted the salt from the sea air. “Thank Merlin,” he breathed. Dumbledore’s body rose with his wand, and Harry grabbed onto the Headmaster’s arm. He envisioned Hogwarts’ gates, open and welcoming, and twisted, disappearing without a sound.