Behind her, the door locked. A few moments later, with an almighty crash, it indented - three separate dents, inward, each curved like a skeletal forehead and baring teeth that left pockmarks in the leaden gate. She decided that it might not be wise to stick around. Cerberus was, after all, traditionally - mythologically, in fact - not an animal anypony wanted to stay around for long. Not only that, but there would surely be someone along to tell her to come out, and damn them, they'd probably convince her that this was a bad idea in a few minutes. Instead, she turned and took a few steps down the stairs, one hoof after the other. She'd expected them to echo, but they just clattered, leaden - a sound like a loose paving slab. She had never really gotten used to that feeling.
Around the steps, as she walked downwards, the walls of the cave were filled with jewels - more than natural, far more than could ever be found near the surface. They gleamed, even with the pale light from the candle on her hat. Thankfully, she'd gotten the candle enchanted by a friend, so that it wouldn't melt or scatter sparks. It'd have to last for a while, of course. She'd have to avoid any sort of water, unless she took off her hat first. (Of course, she'd remove her hat before getting into the water anyway. She couldn't get it wet - no more than she could stop breathing.)
The stairs eventually flattened a little, opening to a small stairwell, lit by candles. On the wall, there was a sign, not so much written as carved into the wall, and not so much carved as engraved. In leaden letters:
ABANDON HOPE, ALL YE WHO ENTER HERE
Of course, she didn't. But she did consider her circumstances. Down there - where she could see pale shoots curling from the dark earthen steps - was, presumably, what she was looking for. That for which she was willing to sacrifice so much - her reputation, her responsibilities, and even her unblemished family record. She'd considered and reconsidered, there on the surface - she was a planner, financially and with her job, after all, though not to Twilight's degree of talent.
This had looked like a bad plan in the light, but she had figured the balances and decided that it was the only real way for her to succeed. Twilight had the most influence, but the girl wasn't a rule-breaker (come to think of it, neither was she.) Rarity was subtle enough… but she wouldn't understand why it was so important. And besides - her penchant for wealth wouldn't be good in the depths, here, with so many alluring gems, so many forbidden fortunes. Pinkie Pie…
She shuddered. Bringing Pinkie Pie down here would be like dropping twelve boxes of lit fireworks into a retirement home.
RD - she loved that girl, but she wasn't subtle. She was rainbow-colored. She could think of things less subtle than rainbows, but most of them were either various different permutations of Pinkie Pie or musical numbers. And she couldn't subject Fluttershy to this place.
Going alone had seemed like the only answer in the light. In the stairwell, it felt a little different. She wasn't entirely sure that she had made the right choice. More than that, she would've liked to have someone there. There was no going back, though. Cerberus was at the gates.
So, instead, Applejack kept walking. And, in the quiet of that place, she made her own life, her own sound, with music:
"Thanks to all my good ol' friends, but now I'm sadly leavin',
"Soon I'll come back to the land, so there ain't no use grievin'.
"Thank yer folks fer bein' born, and thank yerself fer tryin'..."
Down into the dark.
"What's the point of livin' long..."
She stopped.
Down into Tartarus.
Down to find her parents, for the Apple Family Reunion.