Foxglove King - The Nightshade Crown #1 by Hannah F. Whitten

Mortem, the magic born from death, is a high-priced and illicit commodity in Dellaire, and Lore's job running poisons keeps her in food, shelter, and relative security. But when a run goes wrong and Lore's power is revealed, she's taken by the Presque Mort, a group of warrior-monks sanctioned to use Mortem working for the Sainted King. Lore fully expects a pyre, but King August has a different plan. Entire villages on the outskirts of the country have been dying overnight, seemingly at random. Lore can either use her magic to find out what's happening and who in the King's court is responsible, or die.

Lore is thrust into the Sainted King's glittering court, where no one can be believed and even fewer can be trusted. Guarded by Gabriel, a duke-turned-monk, and continually running up against Bastian, August's ne'er-do-well heir, Lore tangles in politics, religion, and forbidden romance as she attempts to navigate a debauched and opulent society.

But the life she left behind in the catacombs is catching up with her. And even as Lore makes her way through the Sainted court above, they might be drawing closer than she thinks.