Spooktober short story

Silva holds her brothers hand as she leads him along the beach, the tide is out and there's a low mist in the air. The familiar smell of salt and brine is a deep comfort this late at night.
The evening chill had set in right as she left her family home further up on the rock, she hadn't had time to grab her cloak as she left. Her little brother, Sebil, is sniffling already, she doesn't want him to catch a cold but her excitement had clouded her sisterly instincts.
"Do you see them?" Silva crouches down to Sebil and wipes his nose, his eyes are fixed on the waterline. Sebil shakes his head and tugs at Silva's hand. "I'm cold Silvie." Before Silvia could respond, small splashes break the water. Maybe 50 ft from them. It starts as three, then five, then around fifty splashes cause the water to ripple. The splashes slowly grow in size. "Dolphins!" Sebil cries out in joy, he does a little happy dance as he watches the splashes, his eyes fixed on trying to get a glimpse at the rare creatures. Silva holds out her hand, muttering a soft word before light shines from a pebble she picks up from the beach. It illuminates the white and yellow shade of the sea, and lights up just a second too late to catch the thorny tentacle that swipes beneath the surface.
"Can we swim with them Silva, I really want to!" Sebil beamed as he watches the splashes, and starts to imitate the dolphin noises he was taught they made. "You'll freeze Sebbie," Silva says with a smile, "lets go get mother, she'd love to see this." Silva turns to walk back up to the town, the light turning with her. The shadows it casts seem longer now.
A scraping and a silent echo. The splashes stop. Silva had let go of her brothers hand when he had danced in joy just a second ago. Now he is gone. Silva walks a few steps before realizing Sebil is no longer with her, then turns. Instead of the smiling face of her little brother, Silva looks into an abyss of tangled twisted flesh that jolts and shudders silently in the water, and out of it. It was too fast to be explained, and if anyone looks past this arm, it would have only found an abyss of tendrils and thousands of desperate, soulless eyes.
The creature's arm towers over her in a matter of seconds, cowering only before the light. Silva finds herself frozen in horror, a thousand thoughts and all she can focus on is 'Where is Sebil?'. A thousand thoughts and one clear answer Silva cannot or will not consider.
Dripping with water and a viscous slime, a throbbing, breathing coil of tentacles unravels and reforms. It pulses and writhes, striking out but unable to reach her. It withers in the dim light. Its form becomes shadow when it gets too close.
Silva stares up into the creature.
From a thousand eyes, it stares back.
Distantly, a bell alarm begins to chime. There is a code for the tolls. Five for a funeral, seven for a missing person; Thirteen chimes for doom. The bell ringer will never reach the final toll. Shouts ring from the cliff edge above, the town wakes in a moment - it is still a moment too late.
Light sputters as Silva tries to concentrate on her spell, on her footing, on anything other than her brother. She runs for the town not a three minutes walk away.
Huts line the lower beach, those that could not afford to live on the rock above, people that already start to run for higher ground. Behind Silva, the sea boils over. More tentacled arms spill from the depths, rolling and dragging themselves across the sand - ooze and slime marring the sand. The smell of rancid, rotting flesh and oil pours into the air. It makes Silva gag as she runs, she screams incoherently at the other townsfolk, 'Light.' she thinks she says. 'It fears light.' she thinks she says. Instead, broken sobs and syllables spill out.
Distantly, in the very back of the townsfolk's minds, they remember a myth. The Sea Maiden, her fall, and the broken children that she could not save. No one knew that they had become this however, this amalgamation of flesh and sea. This creature is of the townsfolk's' blood, and now it wants more of what once made it human.
The tendrils start to strike, each arm a mess of spiked tendrils that open into a thick maw of slime and acid. Silva keeps running, keeps her light alive. She is doing everything right to stay alive. It is still not enough. Silva is young, short, she could never run as fast as her brother, as fast as the others in her town.
A tendril of the arm lurches forward, striking her leg with a sickening force. The snap was heard even above the echoing screams of the townsfolk. The first wave of pain hit Silva hard, stumbling her light spell drops. She howls in pain as thorns pierce her broken leg, wrapping and dragging towards the abyss at the center of the arm. It takes just two seconds for Silva to be dragged into the arm, the tendrils wrapped and encasing her in a pitch black, stale darkness.
Trapped inside the coil of thorns and black tendrils, even the most perceptive would not have seen the flash of fire from inside those arms - as Silva, trapped in darkness, summoned fire that burned and burned and did not effect the tendrils at all. It only incinerated what was trapped inside: Silva and the broken, unconscious body of Sebil.
The fireball does not kill Silva. She lives for another hour, maybe two, as the slime inside the arm thickens and surrounds her. Slowly, it drowns her. Slowly, she dies not knowing where her brother is. Unable to see his burned form next to her, unable to realize she had ended him herself. She had saved him from the slow death she now has. Dimly, as Silva dies, she hears the howling of the creature and hears the screaming of her town silence. She swears she could hear her mother for a second, calling for her and Sebil. The slime in her throat does not let her answer. Slowly, the pain of her shattered leg fades and Silva drowns. Her body and her brothers body left in the endless darkness of this creature to decay. 
When the sun comes up the next morning, a heavy mist lies on the town. The rubble of houses and scores of blood the only trace of civilization ever being here. If someone were to look, the only true remnants of a living being is a small book hidden under a child's pillow. It is titled: A little adventurers guide to dolphins.
Prompts used: Vanish, Abyss, Misfortune, Hunt, Chasm, Drown, Thorn, Echo, Howl, Darkness, Broken, Slime, Shadow, Shatter, Mist