So I was going to answer this with a drabble but it kind of turned into a oneshot. It’s an S4 series finale AU. Note, this has not been edited.

“I can bring Jeremy
back, I can keep him here.”

“No, you can’t!”

“Elena needs him. This
was always the plan. Jeremy wasn’t supposed to die.”

“It was the will of
Nature. There’s no magic in this world that is strong enough to challenge it”

“I have every magic! I
have the spirits!” She started the spell. “Phasmatos revenios un Animum.”

“Stop it!”

“I have expression!”

“Stop it, Bonnie!”

“I have the darkness.”

“Bonnie, don’t.”

“Phasmatos revenios un
Animun. Phasmatos revenios un Animum. Phasmatos revenios un Animum. Phasmatos
revenios un Animum. Phasmatos revenios un Animum…”

Bonnie continued to chant with flames roaring all around
her. So focused she didn’t notice the ground beneath her opening. Before either
he or Grams could do anything about it she fell into a pit of darkness.

The flames disappeared and the only thing left in Bonnie’s
place was the dirty ground.

“Bonnie? Baby?” She’d been tortured in the spirit realm but
none of it compared to what she was feeling right now. Her granddaughter had
disappeared without a trace and she had no idea how to find her.

---

The fall was shorter than expected—granted she didn’t expect
much. She couldn’t help but compare herself to Alice in Wonderland when she
fell down the rabbit hole, but Alice was met with much more pleasant beings.

His name was Kai and she’d fallen into his realm. She’d
heard the stories as a kid, how he had committed the ultimate act of sin and
murdered his siblings. That the act was so atrocious that nature had created a
realm all for him, the only other beings to come through were the dead who
crossed over. Why she was there was a mystery until he admitted to taking her.

The first emotion she felt was fear. If he was the man she
had heard of she knew she didn’t stand a chance. The next emotion she felt was
anger. With all the magic she had in her veins she killed him. First by
impalement, then with fire and then variants of the two until she realized she
was wasting her time, he could never truly be killed, not here at least.

Soon anger turned into curiosity.

“Why did you take me?”  

“Loneliness.”

“But why me? If you were lonely you could have taken
anyone.”

“Because I wanted you.”

“What makes me so special?”

“Maybe the question you should ask is what doesn’t? You,
Bonnie Bennett, are the most special
woman I’ve ever observed, I wasn’t letting that go to waste.”

---

Three Months Later

Mystic Falls was in complete chaos, Bonnie never closed the
veil. Every day was like living in A
Christmas Carol except the ghosts were a lot more pissed off and had a lot
of axes to grind. Seeing their dead loved ones weren’t
worth the carnage it had caused in its wake—they were there to collect and boy
was there a lot to collect. They needed Bonnie and they needed her ASAP.

They learned from Sheila that she had disappeared, may have
even been dead, but a hot tip from the Gemini coven had told them that she was
in their approximation of hell. The weakened veil had allowed Malachai to walk
on earth once more and he had coveted the last Bennett witch. They had no idea
what he wanted with her—there was no way for him to leave, nature had spoken—but they knew it wasn’t good. No one
from the living was supposed to be down there.

“I don’t know about you, but I don’t like the idea of my
best friend being stuck with some psychopathic killer. God, knows what he’s
doing to her down there,” Caroline chimed in. They had weekly meetings—outside
of Mystic Falls—about what to do about the veil.

“I don’t like it either care, but we have bigger things to
deal with right now,” Elena said.

“Which we need Bonnie
for.”

“Who we have no way getting to. We need a witch, plain and
simple.”

“Wow, why didn’t I think of that? It’s not like every witch we’ve come across wasn’t powerful enough
to do the spell.”

“Well, there has to be
one! Bonnie was an untrained witch there has to be at least one witch out there
who can do the spell.”

“And is willing to help us? Witches don’t like vampires,
Elena.”

“Well, Bonnie helped us.”

“Because she was our friend.”

“Alright, we’re getting nowhere arguing,” Stefan cut in.

“I’m all ears, Stefan.
What do you suggest we do?”

“Caroline’s right, we need Bonnie, but there might be a way
to get to her. If there’s a way to the Otherside there has to be a way to Hell.”

“I might know someone,” Damon said. He was unusually quiet lately
mostly because this veil problem affected him the most. It put him in a
permanent contemplative mood.

“Who?”

“Don’t worry about it.” He got up and left the room leaving
five very confused people behind.

---

“Bonnie, Bonnie, Bonnie, stubborn, stubborn Bonnie. Once
again I’ve prepared the perfect meal, and once again you refuse to eat. Do you
know how excited I was to finally cook for someone else? Just one bite—it’ll be
the most delicious thing you put in your mouth.”

“I’m sure it’s very
delicious, but I’ve heard the story. You and I both know what’ll happen if I
eat that. I want to go home, Kai.”

He groaned. “What’s with you and home—it’s not like there’s anything there for you.”

“My friends are there.”

“Are they? I’m sure they’re doing just fine without you.”

“And what would give you that impression?”

“Nothing really, they’re probably dead by now, but if the
only thing standing between them and death is you they were probably meant to
die.”

“How can you say that like it’s
not a big deal? Have you never cared for someone?”

“I care about you.”

“If you cared about me you’d let me leave.”

“But if you leave you’ll die.”

“Says who? Who says I’m going to die.”

“You’ve already died, and if I hadn’t grabbed you, you would
have died again. So, you see I can’t let you leave—you can’t defy Nature and
it’s coming for you.”

She got up from her seat at the immaculately decorated
table—he always tried to impress her—and walked over to his seat at the head of
the table. She took his face in her hands and smiled gently at him. He couldn’t
help but feel comforted by her—he was finally getting through to her. “You are
so, so sweet, but-” She paused. “Don’t tell me what to do.” Her tone was
anything but sweet.

“Ugh, when are you going to learn the difference between
someone helping you and someone trying you control you? Because I gotta tell
you, you’re shitty at it.”

“Excuse me. You don’t know anything about me.”

“Except I know everything about you. I’ve been watching you
for a very long time.”

“That’s not creepy at all.”

“If I could have I would have talked to you.”

“And I would have avoided you every time.”

“And I would have been proud of you for it.”

“You make no sense sometimes.”

“I like you. You’re fierce. Take no nonsense from anyone,
and you’re more powerful than you’ll ever know. I’ll admit, you don’t belong
her—you’re here because I’m a selfish bastard—but you also don’t belong there.

---

Damon knew exactly where to find her. She hadn’t left since
Bonnie’s disappearance. At first, she
just kept waiting for her to come back, but now—knowing where she actually
is—she still can’t leave. It’s the last place she saw her granddaughter.

“After being dead for a year this is how you’re spending
your time among the living.”

She sighed exhaustedly. “There’s only one person I want to
spend my time with—nothing else matters.”

“What do you say we get her back?”

“I say, what’s your angle? You and I both know you don’t
give a damn about my granddaughter.”

“I’ll admit, my feelings for Bonnie aren’t pleasant, but she doesn’t deserve t be
in hell.”

“And?”

“We need her.”

“I’m not helping you kill my baby.”

“Who said anything about killing her? I need her alive,”

“Do you know what she was doing the night she disappeared?”

“No.”

“She was trying to bring Jeremy back, for Elena. As mad as I am about her being I the underworld I’m glad
he took her because if he hadn’t she would be joining me on the Otherside well
before her time. I want to get Bonnie back but I want her to live a long happy
life and she can’t do that with Elena around—she sacrifices too much.”

“I get what you’re saying but you and I both know this veil
needs to come up and not just to save my ass. You’ve sad it yourself—nature
always finds a balance and it’s been absolute chaos for months. This isn’t
going to end well for anyone.”

A long silence sat between them before Sheila spoke again.
“Lucy, my sister’s daughter, she should be able to help you. Although I can’t
guarantee she will.”

---

It had been four months. Four months since she’d seen her Grams, four months since she’d seen her
friends, four months since she’d seen her dad and though she missed them—it got
easier and easier to be apart from them. The more her affection grew for Kai,
this place, even his dog, the more tempted she was to give in, but she held
strong.

They’re in the middle of a game of Monopoly when a new soul
entered. She was winning much to Kai’s annoyance—he’d had years of experience
and never once had he lost this game—she must have been cheating.

“Aren’t you going to get that?”

“No, I’m busy.”

“Someone needs to tell them what’s going on. They must be so
confused.”

“So, they all are, doesn’t mean I have time for them.”

“If you’re not going to greet them then I will.” She got up
in a huff leaving Kai alone with the Monopoly board.

She met them at the river—it was a male. She expected a
completely a terrified soul but found them to be pretty calm.  When she was closer to him she realized she
knew him and her stomach dropped.

“Matt?” she said teary-eyed.

“Bonnie!” he shouted excited.
“I found you.”

“What are you doing here? The only people who come down here
are-” she couldn’t finish her sentence thinking about his fate.

“I know and I’m technically dead, but your cousin Lucy is
going to bring me back.”

“What?”

“I came to get you.”

“But how?”

“Just take my hand.”

“Take his hand, Bonnie.”
Kai’s voice startled her out of her reverie. She turned to look at him. “It’s
what you’ve been asking for, for months. Take it.” He sounded eerily calm to
her.

“But you don’t want that.” She ran over to him to gauge his facial expression more.

“It’s what you want.” His expression never changed—it was
completely neutral. She could leave—the decision was all hers.

“Bonnie, we have to go soon—I don’t have much time before
she brings me back.”

She’s still staring at Kai. “You heard the man.”

“Will I ever see you
again?”

“You know I can’t leave, but here-” He grabbed her hand and
put something in it. “Something to
remember me by.”

She looked at her hand and found six pomegranate seeds. He
was truly giving her a choice.”

“Bonnie!” Matt shouted from the other end. “We have to go.”

Without even thinking about it she downed the seeds and ran
over to Matt’s side. Before she grabbed his hand they gave each other knowing
smiles. He couldn’t leave, but she could come back. With a small wave, she took Matt’s hand. Everything went
bright and then she was in her living room surrounded by her friends and
family.

“Bonnie!” Caroline was the first to run over and hug her.
Bonnie was too overwhelmed with everything to acknowledge anything. The entire
gang was there: Stefan, Elena, Damon, Tyler, Matt, and Jeremy. Her dad was even
there, but that didn’t matter because she found the one person she’d missed
most.

“Grams!” She wiggled her way out of Caroline’s arms and ran
to her grandmother. “You’re still here.”

Grams gave her a knowing look. “You never put the veil up like I told you.” Bonnie’s eyes went wide at
this. “Don’t worry baby, Lucy’s got it covered. Just worry about yourself for
now.”

“But-”

“No buts, you’ve been in the underworld for four months, you
need rest and probably some food, you didn’t eat while you were there, did you?”

She had a mischievous smile when she replied, “Of course
not.”