Death Vigil

From the introduction:

Our story begins with a woman named Clara Jenkins, who died at the hands of her Necromancer boyfriend. But when The Grim Reaper showed up, instead of passing on, she is recruited into the Death Vigil - a group of fighters under the Reaper's command who protect the world from the forces of evil.

The forces of evil are the Necromancers, humans who made a pact with the Unlivings, eldritch abominations from the void outside the universe. In exchange for summoning and feeding their abominations, Necromancers become immortals. A Vigil can only be killed by an Unliving while a Necromancer will lose his powers, his soul and become a vegetable, if armed by a Vigil.