the_debt_of_time

When the jar of fire moved closer to him, Sirius caught it and held it above his head,
desperately searching but not finding her within reach. His attention turned downward as
the water reached his neck, and his gaze focused on a large shadow in the depths below
and the glimmer of something shiny and gold. He took a deep breath just before the water
came up and over his head, sealing them in as it touched the roof above.
Lighting his way with the jar, Sirius swam down in search of Hermione, fighting
against the current that tried to carry him away. His eye caught another glimmer, but instead
of the determination he had felt when he spotted the cup at the bottom of this watery
tomb, the sight of the Sword of Gryffindor made his heart sink. He had last seen it in
Hermione's grip.
Panic flooded him, and he instinctively reached for the hilt of the sword, gripping
it tightly. A nearly overwhelming part of him wanted to find Hermione, to save her, but the
very small logical part of his brain fought back with a vengeance. The Horcrux was to
blame, and even if he found her, he could not hope to save her while still being drowned
by a bloody goblet.
Sirius swam, moving swiftly despite the fight the Horcrux was putting up, as well as
the awkward length of the sword in his hands. He caught sight of the cup and—even
though the water pushed in against his chest, forcing what little oxygen was left from his
lungs—he brought the tip of the blade up and thrust it down, stabbing through the centre
of the object.
The cup began to shake violently as a black liquid seeped out, swirling into the water
around it.
Sirius propelled himself backward to avoid the dark liquid. He struggled against the
water around him, trying to kick his way to the surface of the sinking water level, catching
only a brief glimmer of hope in the form of the broken rubble at the passage's exit where
cracks in the cave allowed the water to swiftly escape.
When his head finally broke the surface, he let out a loud gasp, taking in air as deeply
and quickly as possible, and coughing up water that had forced its way down his throat
during the struggle.
Catching his balance as his feet hit the ground, the water finally having drained
enough to allow him to touch the floor, Sirius scanned the room, searching for Hermione.