Here are my thoughts. Hope they help!
Mora Aresh healed the fractured world of Orenos through cunning, pragmatism, and old-fashioned charm. Nice opening--gives me a good sense of your character. It took her thirty years to undo her mother’s failed invasion, a mystery that’s plagued her since she was a young girl. This sentence confused me a bit: did her mother invade from another world? If this was a world war it might make more sense to say "her mother's failed war." Plus, an invasion doesn't seem like a mystery, unless you mean WHY she invaded is a mystery. At the height of her power, Mora gave everything (should this be present tense? The rest of the query is in present tense.) up to pursue her last link to her mother: a tablet linking Mora’s worldfaring race to Orenos, centuries before her mother's invasion. Chasing a strong lead half way across the world, Mora finds only dust and ash. Instead, something finds her.
The Ivory, an insidious force that’s plagued races across the cosmos for millennia, infects her crew. Mora barely escapes with her life; she returns to her former stronghold seeking answers from her successor and cousin, Aratha. Aratha greets her with a disturbing claim to have
surprise: she’s mastered the Ivory and found the means to overcome its influence. Aratha’s erratic behavior says otherwise, and Mora narrowly escapes conversion at Aratha’s hands.
Grief-stricken at Aratha’s betrayal, Mora casts aside her past. She becomes an Outrider, a mercenary that carves her path according to Mora’s faith. Cleansed of her failures, Mora launches a counterinsurgency to reclaim her legacy. But grudges don’t fade and Mora has to prove to the world she isn’t a tyrant like her mother or cousin. As Mora wages skirmishes against Aratha’s acolytes, the Ivory’s song spreads and Mora uncovers a disturbing truth: she can use the Ivory without losing her will. As blood flows and gunpowder sings, Nice line. Mora questions her motives for her insurgency. She’ll have to decide between ending Aratha’s reign and discovering the Ivory’s purpose for Mora’s ambitions. Whichever she chooses, Orenos will march toward its end. Good ending, though I wish you'd explained why Orenos is doomed either way earlier.
A TYRANT COMES is a 115,000 word dawn of the 19th century epic dark fantasy. Up until this point I hadn't gotten any hint of 19th century technology; it seemed more high fantasy.
This seems like a really interesting story! My biggest critique is I think there's too much going on. I suspect you've given away more of the story than is normal in a query letter (surely it takes us at least fifty pages to get to Aratha's takeover?) Plus there's a lot going on with an intricate plot and morally complicated characters. For a query letter, I think it's okay to pick one or two things to focus on and write out the rest. Either the first paragraph or the third paragraph look like separate query letters, and you could turn either one into it's own letter. I think either letter would be cool and fascinating.
Hope I've helped! My query letter is here.