Witches and War. Warwitch Cycle, #1

In 1918, as the Great War grinds to a bloody stalemate in the trenches of Europe, the British War Office authorizes an unthinkable plan: recruit the last surviving members of the ancient Coven of Hollowmere to turn the tide. Bound by a centuries-old pact to never meddle in mortal conflicts, the witches agree, on one condition: they will fight not for nations, but for the dead. As the coven arrives on the frontlines, they weave spells that pull ghosts from the no-man's land fog, phantoms of soldiers who died minutes earlier, forcing them back into their bodies to fight once more.
But the magic comes with a cost: every resurrection corrodes the resurrected's soul, turning them into something not quite human. When the witches begin to see visions of a future where the war never ends, they realize someone among them is deliberately prolonging the bloodshed to feed an ancient, hunger-starved deity. Now, trapped between the demands of generals, the screams of the undead, and the whispers of a god older than war itself, the witches must decide if they will break the pact, betray their allies, or burn the world clean.