In LOTR with Harry Potter system

The black arrow failed to pierce Smaug, it was incinerated mid-flight by the dragon's fire and fell as molten slag.

A wave of despair rippled through those watching.

"Bard, there's still a chance!" Sylas shouted urgently, soaring down on his broomstick to meet the stunned archer.

Bard looked up, his hands trembling around his now-empty bow. "But... that was the only arrow. The only thing that could pierce a dragon's hide. What else is there?"

Sylas reached into his spatial pouch and pulled out a gleaming spear, its silver shaft pulsing faintly with ancient power.

"This is Aeglos," he said solemnly. "The divine spear of Gil-galad, High King of the Noldor. It once pierced Sauron's very armor. Its power and sharpness are unrivaled. The length is close enough to match the black arrow, use it."

Bard caught the spear, awe flashing across his face. He could feel the weight of history humming within the weapon's core. 

Yet his expression darkened with doubt. "But the dragon's weak spot is no longer exposed. He covered it with molten arrow metal. Even with this... I fear there's no opening."

"Then we don't aim for the chest," Sylas replied, eyes sharp with purpose.

"Not the gap?"

"No. This time, strike his wing. Sever it. Bring him down so he can never fly again."

Sylas met his eyes. "I'll give you one opening. Just one. Can you take it?"

Bard inhaled deeply, nodding. "I can."

With no time to lose, Sylas turned and rocketed back toward the battle.

Far above, Gandalf clung to Smaug's back, one hand gripping Foe-hammer buried in the dragon's back, the other unleashing radiant pulses of light to blind the beast's eyes. The dragon flailed midair, his path unsteady.

Sylas streaked through the sky, his wand flashing with a sharp Confringo! The wave of explosion slammed into Smaug, making him snarl and stagger mid-flight.

Coordinating seamlessly, Sylas and Gandalf drove the dragon back toward the town, maneuvering him, herding him, into Bard's line of fire.

Now came the final step.

"Imperio!" Sylas bellowed, pouring every ounce of magic into the spell.

He felt it at once: a pressure like trying to bend a mountain. The dragon's soul was vast, ancient, and powerful, like molten rock wrapped around a burning star.

Even with the full force of the Unforgivable Curse, it was like trying to pull a tidal wave with a thread.

However, even that brief moment of distraction was enough, Smaug's focus faltered, and his flight turned sluggish.

"Now, Bard!" Sylas roared, veins bulging as he strained to maintain the Imperius Curse.

Bard, already braced atop the tower, rested Aeglos, the divine spear, against the groove of the great war crossbow. The instant Sylas's voice rang out, Bard loosed the weapon with perfect timing.

The spear shot forward like a streak of silver lightning, slicing through the air with a shrill whistle. Aeglos struck true, piercing through the hardened scales at the joint of Smaug's right wing, the connection between muscle and bone.

A piercing scream tore from the dragon's throat. Smaug writhed in the sky, then dropped like a stone, a bird with a shattered wing, spiraling downward at terminal speed, Gandalf still clinging to his back.

With a colossal crash, the dragon's massive body slammed into the waters of Lake-town, unleashing a tidal wave that swept boats and debris across the harbor.

Even submerged, Smaug's sheer size left half his body still above the water, scales glinting in the firelight.

Atop the dragon's back, Gandalf stood firm, untouched by the fall, his robes rippling in the rising steam. With a cry, he raised his staff and struck again and again, releasing bursts of light and shockwaves that forced the beast further into the lake's depths.

Smaug thrashed, trying to regain lift, but the wound in his wing left him unbalanced and grounded. Every desperate beat of his remaining wing only sent him in spirals or crashing back into the water.

"No! This isn't how it ends!" Smaug howled in despair, his voice thunderous and ragged. 

But Sylas had no intention of giving him another chance.

He raised his wand and shouted, "Glacius!"

A fierce, unnatural cold erupted from the tip of his wand, spreading across the lake in rapid sheets. The water around the dragon began to freeze, crystalline and thick, rising up like jagged spears of ice to trap Smaug where he lay.

The dragon roared and thrashed, flames erupting wildly from his jaws, melting the ice as fast as it formed. In his madness, he turned his fire even upon himself, burning his own hide to prevent being sealed away.

Gandalf leapt clear, his cloak smoldering at the edges as he barely avoided the inferno.

The flames spewing from the dragon's mouth were nearly unquenchable. Even submerged in the lake, they danced across the surface, scorching the water and spreading outward like living fire, inching dangerously close to the shores of Lake-town.

"Sylas!" Gandalf shouted over the roar of boiling water and panicked cries, "We must stop him from breathing fire, Lake-town won't survive another blast!"

Sylas understood the threat all too well. Without hesitation, he raised his wand high above the lake and called out,

"Carcer Aquae!"

With a surge of magical energy, a colossal whirlpool formed, spinning furiously until it converged into a massive sphere of water that encapsulated Smaug, lifting him from the lakebed and suspending him mid-air inside the rotating vortex.

The moment the dragon opened his jaws to unleash another torrent of fire, the spell responded, gushing water surged directly down his throat, choking off his breath and dousing the flames before they could ignite.

This was no ordinary spell, it was Carcer Aquae, a powerful spell Sylas had learned from one of the tomes provided by the system. He remembered it vividly from Dumbledore's duel at the Department of Mysteries, where the headmaster had used a similar spell to imprison Voldemort in a swirling sphere of water."

But conjuring such a spell on the scale of a dragon was beyond anything Sylas had ever attempted. Thankfully, the surrounding lake offered him a limitless source of water to manipulate.

Inside the spinning sphere, Smaug twisted and flailed violently, tossed in every direction like clothes in a raging tumble dryer. His attempts to breathe fire were drowned out, each roar met with a flood of cold lakewater crashing into his maw.

Sylas clenched his jaw, his brow furrowed in deep concentration. He poured his magic into the spell, weaving the currents tighter and faster. Though his magic reserves held, the mental strain began to wear on him. Sweat streamed down his temples, and his limbs trembled under the weight of sustaining such immense power.

Just as he felt his mind begin to waver, a sudden wave of warmth and energy surged into him, a gentle flame that did not burn, but kindled strength.

He turned and saw Gandalf, staff raised, and the Ring of Fire, Narya, glowing with radiant light on his finger. The Elven ring, bearer of hope and perseverance, infused Sylas with renewed vigor.

"Keep going!" Gandalf called, voice steady, "Narya will lend you the strength you need!"

Seconds dragged into minutes.

Smaug's massive body thrashed slower and slower.

Then, he stopped.

Suspended within the prison of water, the dragon hung limp, motionless, limbs drifting like a corpse in the tide.

Neither Gandalf nor Sylas moved. They waited, breath held.

Smaug was ancient, a creature of ruin and flame. To assume he could drown so easily was to court death.

And true to form, his eyes snapped open once more, burning with malice and cunning.