1880s London: After being confronted with her dead, abusive father at a séance, an American assassin ignores her mission preventing a looming war, to seek revenge on the spiritualist responsible.

I see this story as the spawn in an unholy biological union between Alfred Hitchcock and Dario Argento in sci fi drag. Tear it up.

Interesting, but I’m afraid I’m gonna need a little bit more. What did her father tell her? Why is the spiritualist responsible for this? Clear some of that up for us, the reader, and hook us please.

London 1880s: After confronting her abusive father’s spirit at a séance, an assassin battles unearthed trauma while seeking the spiritualist responsible in order to enact revenge.

I like this version better.  Do the dots need to be connected more? It doesn't matter what her father said. Not for the logline anyway. Spiritualists host seances, is that no enough?

Should it be "her revenge"?

The dots definitely need connecting more imho.
Connecting through a seance sounds voluntary so why blame the spiritualist? And the link with your protag being an assassin? Sorry, I'm not getting the story elements at all.

London 1880s: After an unbeliever is confronted with her abusive father’s spirit, she battles with the unearthed trauma while hunting the spiritualist responsible to get her revenge.

Two bird with this change.

Should it be "unearthed childhood trauma" or is that implied?

Yeah. It’s like, did the spirit of her abusive father bring down some sort of evil with it that manifested itself and caused the daughter some major problems? Was the spiritualist in on it? Did they have some sort of ulterior motive? That’s what it sounds like. Yes, a little vague.

Unfortunately people get traumatized by people in their childhood all the time. Even in a world where it is possible to manifest ghosts. People are haunted by memories in real life. Just because the world is a bit more fantastical doesn't erase that. In other words, the woman's trauma is not supernatural, and could happen in our world. Yes, the supernatural elements come heavily into play throughout the conflict, but those forces are not the driver.

It's steampunk, but it's grounded.

So how do I convey the above in a logline? How about a lognovel?

1880s London: At a séance, an unbeliever is forced to confront her dead father. She struggles with memories of his past abuse, misplacing her self-hatred to seek vengeance on the spiritualist who brought her father back.

This feels like describing King Kong as a movie about an ape that falls for a tourist. While technically true, it captures none of the story or spectacle.

A 19th century American gun for hire, on a job in London, forsakes her paid hit to seek revenge on the spiritualist who conjured up the abusive spirit of her dead father.

Language wise it's fine, but it suffers from the same issues other people have brought up.

I think I've got all the details in this attempt, but it's sooooo long.

A 19th century American gun for hire, on a job in London, is forced to face her childhood traumas by the ghost of her abusive father. Can she overcome her personal demons and misguided desire to reap vengeance on the spiritualist who conjured the spirit and complete the mission before her impatient paymasters hunt her down?

An assassin’s mission is interrupted when she participates in a séance, unexpectedly confronting her dead father. She struggles with memories of his past abuse as she ignores her mission, instead pursuing vengeance upon the spiritualist responsible.

Unbeliever doesn't work because that's not what she is throughout the story. She's an assassin. She kills people. Anyway, this is a fat 36 words and I think it properly closes off the previous criticisms. Does it?

Imagine writing a novel and struggling this much over one or two sentences.

Quoted from Hallucigenia An assassin�s mission is interrupted when she participates in a s�ance, unexpectedly confronting her dead father. She struggles with memories of his past abuse as she ignores her mission, instead pursuing vengeance upon the spiritualist responsible. Unbeliever doesn't work because that's not what she is throughout the story. She's an assassin. She kills people. Anyway, this is a fat 36 words and I think it properly closes off the previous criticisms. Does it? Imagine writing a novel and struggling this much over one or two sentences.

Read anything by Fyodor Dostoevsky, and you'll see a novel writer who labored over every sentence just as much.  Anyway, I added this bit, "...before her impatient paymasters hunt her down?" without even knowing if that was in the story, because she needs to have some kind of ticking clock she has to beat.  Some threat she has to overcome.  Her father isn't a threat, just a bad memory, and she is a threat to the spiritualist, but what is she working against?  My only thought is the guys who paid her to off someone are getting ticked because she's blowing off her job.  So, while she's dealing with all that other junk, she has some heavy mob Irish Catholics after her because she didn't off Queen Vic like they wanted.  It's what Guy Ritchie would do!  

Sincere due respect:

BUT

I have read all the posts here and I am still somewhat confused. You need to up the game in explaining what is going on here (with this story).

We have an American assassin – and – someone on a mission to prevent a looming war.  So now I have to guess that it is one in the same (i.e., her mission is to prevent the war by assassinating a specific individual) Reading between the lines, she sounds like a CIA type agent on a mission to take out a dictator or something.  AND – the stakes are – if she doesn’t end this distraction with her father’s ghost, war will break-out.

The revenge factor is muddy. Not clear why she is seeking revenge against the spiritualist – so again I am going to guess that she did not hire this spiritualist – that instead she was somewhat attacked by this person (i.e., this person intentionally released this dysfunctional spirit without the knowledge or consent of our hero).  

London 1800s: An American assassin on a mission to kill (WHO IS IT) in order to stop a looming war is derailed when a Spiritualist covertly summons the ghost of her abusive father.

Maybe we can get it better by answers to these questions:

Who does the assassin work for?

Who specifically is she supposed to kill?

Did she hire the Spiritualist – if so – then why is she mad at this person for doing what they were paid to do?

If not – who hired the spiritualist?

Is the ghost of her father real – or some kind of trickery?

Is the ghost there throughout the story? i.e., is her haunting our hero while she is on this mission or was it a one time apparition?

What are the consequences of her not completing her mission – is there a time clock to this? E.g., does she have a week to get it done?

What happens to her if she fails to complete the mission?

This is a tough one. I think I'm close, but I have questions.

Like Dave alluded to - if she's an assassin sent to prevent a war, where does this seance come in? Seems out of left field. It tells me there might be a logic concern early in the script.

And the war itself - you say "to prevent a looming war". I looked it up and in the 1880s the British fought the first Boer War, which obviously did happen. Is she involved in this? Or did she prevent the war? If so, between which two sides?

And loglines should really be one sentence. They can be compound sentences, but the longer they are, the more the reader loses the plot along the way. Here is my attempt, making some assumptions:

An American assassin, embroiled in the First Boer War, confronts her dead father at a seance and seeks revenge on the spiritualist.

Hope this helps.

AJR