Daphne braced herself as the dark azure, almost black flames burst into a cloud of hissing steam that flooded the corridor, obscuring her vision. The steam was not hot to the touch, but deathly cold instead. Ominous shadows wavered in and out of sight beneath its cover, cast by the surviving blue flames flickering dangerously through the haze…
"Protego Diabolica," she murmured.
The dying flames revived, burning with renewed vigour as Daphne waved her wand to weave them into forming a protective wall before her. Even though Voldemort was still the superior duellist, she did not dare to be complacent. Although Oleandra appeared to be recycling her old tricks from their duel in the Department of Mysteries, she had time and time again proved herself to be an unpredictable opponent.
The sound of trickling water reached Daphne's ears; water was seeping through the gaps between the flagstones and flooding into the corridor, soaking the hem of her robes. With a flick of her wand, she performed a nonverbal Drought Charm, creating a boundary through which no water could pass.
Steam swirled as footsteps echoed closer. An eight-branched sigil burned through the veil of water vapour, glowing with blood-red malice, and underneath it, a single, golden eye whose gaze seemed to pierce through her…
"The Lokk of the Helm of Awe," Daphne said coldly. "I learned of this spell through my host's memories. You chose wrongly. You will draw no fear from me."
Just like Protego Diabolica, Ægishjálmur was both dark and protective magic, a rarity in the Wizarding World. The Helm of Terror drew its protective strength from the fear it inspired in others, which meant that if those affected controlled their emotions, its power would falter.
"Bombarda Maxima!"
A thunderous boom rang out through the confined corridor as Daphne aimed an Exploding Curse at the golden sword floating above the Helm of Awe; she still could not see Oleandra's face through the steam, but she knew the sigil was branded upon her brow.
The resulting shockwave rippled through the passage, ripping sconces from the walls, tearing flagstones from the floor, and dispersing the steam that obscured her sight. The thick curtain of vapour vanished in an instant, revealing a rippling, heptagonal window of floating water reflecting the night sky, behind which Oleandra's distorted silhouette stood.
"Laukaz, Isaz, Hagal, Teiwaz!" Oleandra shouted, punctuating each rune with a hand sign, before thrusting her hand through the water mirror. "The Lake, the Brilliant, the Winter to End All Winters, the Ever-Victorious Lance!"
Sharpened spears of ice burst from the water mirror's undulating surface and streaked towards Daphne, but the icicles melted almost instantly the moment they penetrated the wall of blue flames surrounding her.
Daphne responded by waving her wand in a circle around herself, Conjuring a mighty serpent from the flames and sending it crashing into Oleandra's water mirror. Its smouldering fangs sank into the mirror's frame, causing the reflected image to waver, but just as ice could not overpower fire, neither could fire vanquish water, not when their powers were so closely matched.
"Then, what about this?" Daphne snarled. "Fulgurante Fulminare!"
Blue lightning sparked from her wand and arced across the corridor in an instant with an ear-shattering thunderclap.
"Tinne!" Oleandra shouted, extending a palm towards the water mirror.
The lightning sprang from the mirror to her arm, millions of tiny blue bolts arcing harmlessly across her skin. By using the holly ogham rune in this manner, she could shield her internal organs from shock, but there was no time to waste; she would not be able to contain lightning magic upon her body for long without risking injuries far worse than mere burns.
"Straif!"
The blackthorn rune resonated with Oleandra's Fairy nature, amplifying the power of the lightning rippling across her and sending it back towards its caster with a vengeance, forcing Daphne to hastily raise a silvery shield of magic to block the lightning strike.
"Laukaz, Ingwaz, Naudhiz…"
Recognising the first three runes of the Insigil of Lindorm, Daphne immediately sought to stop Oleandra from casting her spell.
"Piertotum Locomotor!"
Oleandra stumbled as the floor began to tremble. Flagstones rolled free from the floor, walls, and ceiling, forming stone hands that clawed at her, but Oleandra pirouetted out of the way, ducking and weaving between them as Daphne waved her wand like a conductor's baton, directing the assault.
The corridor was warping beyond recognition, converging on her, cutting off her escape routes one by one, and… water started gushing out of the cracks in the walls and ceiling.
"I was hoping to buy a little more time," Oleandra sighed as the corridor closed in, "but you've forced my hand."
All this time, Oleandra had been stealthily flooding the dungeons with water from the Black Lake, but Daphne's destruction of the walls had forced her to reveal her hand before she could accumulate as much water as the dungeons could possibly contain.
A torrent of water crashed through the cracks between the animated stone hands with a thunderous roar, sweeping Daphne head over heels down the corridor and back towards the Slytherin common room, the lowest part of the dungeons.
Thanks to her inborn Protection of the Lake, afforded to her as the Lady of the Lake, which allowed her to ford any body of fresh water unimpeded, Oleandra slipped easily through the currents in pursuit.
"OLEANDRA GREENGRASS!" Daphne screamed, her voice muffled slightly by the Bubble-Head Charm around her head. "How dare you…!"
Oleandra twisted her body effortlessly as a stone serpent came rushing through the turbulent waters, avoiding it by a hair's breadth.
Having accumulated an insufficient volume of water from the Black Lake, Oleandra's torrent soon lost its strength; Daphne had already washed up at the end of the corridor, her green hair undulating agitatedly about her head…
"Avada…"
"THURSAZ!" Oleandra roared.
Daphne's voice cut off mid-incantation as the spell slammed her into the hard wall behind her, but she was immune to all damage; Voldemort had made her so… she snarled and scrambled to her feet, but Oleandra was already upon her. Excalibur in hand, she brought down its golden pommel down upon the crown of her head with a sickening crunch.
Oleandra staggered, her vision swimming.
She had been poisoned; her forearms were riddled with microscopic poison-tipped needles, because in knocking her out, she had touched her sister's green hair…
…but thankfully, she had built up a resistance to her sister's toxins over years of accidentally brushing up against her.
Oleandra took a few deep breaths, muttering, "Saille," under her breath a few times.
She had won.