Basically Twilight x Trixie -story, that I planned to draw. Let's say that this take place between 1 - 3 Seasons before "Magic duel" episode.
Trixie step on the Poison Jokes, that transform her into an sun hating, isolating vampire pony.

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Unable to eat a normal food as it taste bitter in her mouth, and she start to hear pony's heartbeats and having a X-vision to see the blood flows through pony's body. Making her drool for sight.
But quickly shake it off in panic, when she realize what she is doing and run off. Hide herself under the blanket.
Being very confused and scared what is happening to her.

During the evening when ponies are going to sleep, Trixie's stomach was growling even harder for hunger. Want so badly to eat.
A only pony, who could help her right now, was her the most hated enemy. Twilight Sparkle.
With that thought, Trixie sneak into Ponyville, to the Golden Oak Library.
Finding Twilight sleeping peacefully in her bed. She sneak in from the window and walk next at Twilight, attempt to wake her up.
( She need this disease cured so badly. )

Until... Twilight reveal her neck to Trixie.

Trixie's eyes widen as she can smell the blood pumping through Twilight's body. The delicious blood... Twilight Sparkle's blood.
Trixie is drooling over Twilight's neck and stare at it.

She shouldn't. But, maybe just a little wouldn't hurt... she thought. At this moment Trixie didn't care anymore. She was too hungry to wait for cure. She need this now.

Slowly, Trixie lowered herself closer, showing her sharp teeths as she sink them into Twilight's purple skin. Closing her eyes in disgust and fear. But soon she felt comfort like a filly, who's mother was embracing. When she felt the blood filling her hunger, her eagerness fill her even more, when she embraced Twilight's body with her hooves, pressing herself against her. So close that she could smell Twilight's scent. Trixie's cheeks were burning for blush. This new feeling was overwhelming, that a tears were forming into Trixie's eyes.

Even Trixie hated Twilight so much and wanted humilate her... But how could she hate somepony, who has so luscious blood? She wouldn't want anything happen to this nerdy bookworm...
Sucking so much that Trixie fail to notice how cold Twilight was becoming. Realizing this, Trixie pulled her bloody teeths off from Twilight's neck in panic. Looking down in horror if she did harm Twilight.
Quickly pulling the blanket over Twilight, covering her with it and cleaning the blood off from Twilight's neck with her cape.

Staring there for a moment at Twilight, Trixie couldn't help to notice that her cheeks were burning for blush. She was blushing? Why? No way she would blush for somepony like Twilight Sparkle. She was a... enemy.

Soon guilty fill her conscience as she left the library and returned to her hideout. With satisfaction, she felt she was alive again.

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And that's much about it I came up with.
What do you think? How would you continue this?
You're welcome to do it.

6412000
Ok I get poison joke is supposed to cause an opposite effect with those who are exposed to it, but how’s this an opposite thing to happen to Trixie?

6482823
Not sure how Poison joke works exactly. ( Changing the Mane six's bodies anatomically. )
But if it is has an opposite effect on the character's nature, then maybe it work also like TMNT's Mutagen Ooze effect. ( after hit with the Ooze, the being (whether it be a human or animal) gets its DNA mixed with the last living thing(s) it touched, resulting a mutation. )

I imagine a situation Trixie walking in the Everfree Forest, trying find a way from the forest, after hastily running away from Ponyville in "Boast Busters".
Running into a night bat, that fly on her face and cause her fall into the Poison Jokes.
Resulting the bat DNA mix with hers.

And for the bloodlust, I imagine it is Trixie's own ego.
Hunger for fame and glory, mixed with bat's fangs, turned into a hunger for blood. ( If that make any sense. )

***Edit***
Or her blood boiling hate on Twilight Sparkle, turned into a bloodlust.
*Got the hots on Twilight's warm blood*