Even in fantasy, a world must have its own logic. The difficulty fantasy writers have is that in their invented world, they must also invent the logic. If the premise of your world is complicated, then best not to start with too wide a world-view. Start with a detailed scene, and leave the world-building out.

Title: The Thief That Slit Heaven’s Purse
Wordcount: 21 000
Genre: Fantasy
Language: British English
Synopsis: A gifted thief snuck into heaven and stole a piece of divinity from the goddess of civilisation. A decade after going into hiding in a thriving island-city, someone reveals her identity to the arch-priest, monarchs, and traders, who close off the ports and hunt her to try claim her powers.
Text:
The thief made no sound as she entered the cave, so it was not the Lion-Woman’s ears that twitched in alarm, but her nose and every fibre of her hide. A smell unfurled in the darkness. It was the salt of the sea, old copper, and the crisp breath of mountain air. Good smells, but beneath them, the stench of burnt skin crept in, banded with smoke and dried blood. And not just human blood, which was iron-rich and flavoured with impurities. She smelled ichor too: life-giving, golden, and godly. Pure. It did not belong to the thief.
The Lion-Woman raised her giant head off her forepaws. The cave ceiling was too low for her to unfold her wings, not that she wanted to. The world had died, come back to life, and died again since the last time she had left the cave. In the First Dawn, she had been worshipped only below the gods and far above man, semi-divine even. In the Second, she had at least been a legend to those who remembered. In this Third Dawn, she was barely a myth. Those who knew her knew too much of the world, and anyone capable of finding her was capable of causing the Fourth Dawn. Of the seven mortals who had entered her cave in the past millennium, she had only allowed one to leave, and to this day was not sure if she regretted it.
The thief sat down just out of reach, her silhouette a little darker than the overlapping shadows. “Hello again.”
“Unravaller.” The Lion-Woman lay down on her forepaws again, though her tail flicked in agitation. “Were you not here a month ago?”
“It’s been thirteen years.”
“Well, if a fly circled your head and you gauged the time it took, you would call it an instant, the fly would call it an hour.”
The thief sighed. “I’ve come to barter.” The timbre of youth still reverberated in her voice, but it had been tempered. It was no longer the ripple of water but of steel.
“Oh?” said the Lion-Woman. “Bartering? How low you’ve sunken since you solved my riddle.”
“I didn’t solve your riddle, I traded for the answer. I need to buy back what I bargained with.”
Irritation bled from the thief. Oh, it was enough to bristle the Lion-Woman’s fur. For one so versed in the art of trade to show even the smallest hint of desperation… It was beyond delicious.
Her wings twitched. “Oh, certainly, certainly. But the answer was not cheaply sold. With it, you stole into heaven and touched the blood of Sian.”
“And I gave you my name in return.” The thief’s voice bounced off the low ceiling. She sighed. “I want it back.”
“Why? If I gave you your name back, Sian could find you. And she would not kill you, oh no. An immortal knows worse punishments than death, and she has delighted in them against mortals with a tenth of your offences.”
“Just say you will trade. Please.” There was a tremor on the last word, as if the thief’s throat had tightened while saying it.
Desperation and now fear? These were not things easily felt by the likes of the thief. One did not possess the mind to puzzle out divine curses, the resolve to scale Himmel’s back, and the iron-clad nerve to steal from goddesses and then suddenly fear. Whatever had brought the thief here, the Lion-Woman was suddenly sure she wanted no part of it. And yet…
“What would you trade for your name back? What could you possibly have that is greater than your very essence?”
“Freedom. An hour’s worth.”
The Lion-Woman scoffed. “I can leave this cave anytime I wish.”
“Not without the gods smiting you,” the thief said. “I could shield you long enough for you to unfurl your wings and dip your claws into ocean water, or to soar high enough to see the snow peaks in the west.”
The Lion-Woman did not consider it; she had had centuries to do so. A few more moments would not change her mind.
“I do not want to sniff a meal I cannot eat. But…” She paused a moment, then smiled. “I believe you cannot tell a lie.”
The thief’ silhouette was very still. “I could — if I tried. I choose not to.”
“Oh, don’t be so noble with it!” The Lion-Woman roared. “You touched Sian, Goddess of Luck and Civilisation. You are bound to the magic of commerce and trade, travel and guest-rite–” Her tail thumped the cave floor. “–language. What would happen if you told a lie?”
“The same thing that would happen if I kept a coin longer than a day, or travelled with more than I could carry on me. Civilisation would splinter.”
The Lion-Woman’s wings ached wonderfully as they touched the ceiling. “Then tell me a lie and I will give you back your name.”
“It is not that easy.”
“And I don’t want it to be!” Another roar thundered through her. “I want you to squirm when you tell it. It must twist your flesh and cost you everything. Your arrogance has earned me the pleasure of seeing that.” She tucked her wings back and curled her tail around her legs. “In fact, tell me the most wicked lie you can muster.”
“A great lie must be buried in an even greater truth,” the thief said coldly.
“Even better,” said the Lion-Woman. “Weave me the tale of how you came to sit here begging. Start at the last point you felt secure in your hubris.”
She felt the thief’s eyes on her. For a long while there was stillness, then the thief seemed to bend low and trace something on the floor. Her finger made no sound against the stone, but when she pulled it back, a rune illuminated the floor with a soft orange glow. It reached up and lined the contours of the thief’s face. The Lion-Woman shuddered.

Editorial comment: I like this opening. It is high fantasy, but shows some flashes of originality and neat phrasing that set it apart from run-of-the-mill swords-and-sorcery openings. It shows a thorough appreciation of the genre, being able to handle the tropes of fantasy so subtly that a reader accepts the trope without question (a thief as a “character class”, casting of runes, interaction between deity and mortal). There are a number of things that might give an agent pause, however.
First is the length. You say this is a “book”, and don’t qualify it any further, but at only 21,000 words it’s not a novel, more a novella. The market for novellas, particularly from unknown writers, is pretty small. Is this going to be expanded to a full-length book, or is the novella the finished article? There’s an off-chance that an agent might know a publisher who is compiling an anthology of short stories/novellas, but I think it’s more likely that if an agent really liked the writing they might react by asking you, “Have you got anything else I can have a look at?” That’s no bad thing, obviously, but it isn’t a straight acceptance.
Secondly, I’m not too sure about the premise. If this thief had stolen divine powers from a God, how did she manage to remain unobserved for over a decade in the naturally confining boundaries of an island city? What were the powers she stole, and what use, if any, did the thief make of them? If she didn’t use them it begs the question, why go to such extraordinary lengths to steal them? If she did use them, how did she remain unobserved? This might become evident as the story unfolds, but there must be logic, even in a high fantasy world.
Of the writing specifically I like the first paragraph, and the third continues the detail of the interaction between the thief and the Lion-Woman, but the second paragraph is weak. Between being very specific (what the thief smells like, and her sitting down “just out of reach” and beginning to talk – all very immersive and scene-setting) you lapse into very passive world-building generalisations – “she had been worshipped“, “she had … been a legend“. I don’t really understand what “Those who knew her knew too much of the world, and anyone capable of finding her was capable of causing the Fourth Dawn” means. The thief has found her. Does that mean the thief is capable of “causing the Fourth Dawn”? That sounds pretty major, but we’re given to understand that the thief is running scared of pretty much everyone. Again, what are these powers, that can shield the Lion-Woman from the gods, yet not protect the thief? If this paragraph was just omitted, the whole scene is far better, but it’s a worry if the underlying premise is faulty.
A minor point: You say that it’s written in British English, but there are a number of Americanisms that to my British English ear sound out of place. “Snuck”, in the synopsis, is an exclusively US past participle, of which “sneaked” would be the BrEng equivalent. Similarly US English generally uses “smelled” where BrEng would use “smelt” (elsewhere you have “burnt”, and not “burned”). It’s also an Americanism to omit conjunctions “hunt her to try [and] claim her powers”. I don’t think it matters unduly, but best to choose a language version and stick with it consistently.

I think this is flawed, for the reasons stated, so it’s not gold star material, but the writing might be good enough, bar that second paragraph, to interest an agent into at least asking if you had anything else more marketable. That would be an achievement in itself.

Thanks for posting.