Mr. Whiskers: Lord of the Void
Fair notice, this is my book. It's free for the next few days, and ranked fairly highly, surprisingly! Here's the Amazon blurb. Feel free to grab a copy! Or not. Up to you. I won't pressure you.
--- blurb ---
Infinite cosmic power. Itty-bitty living space.
To the universe, he is a Class Omega entity of undefined chaos, capable of rewriting reality and staring down elder gods. To Detective Emily Chen, he is Mr. Whiskers: a stray black cat who appeared at a crime scene, eats too much Fancy Feast, and refuses to stay off the furniture.
It is 1982 in Chicago, and the cracks in the world are starting to show. A cult obsessed with an ancient serpent is stealing artifacts, bodies are vanishing from the morgue, and a comet is hurtling toward Earth with an appetite for destruction.
Trapped in a feline body, stripped of his powers, and forced to rely on a skepticism-prone detective and a teenage bellhop with a baseball bat, the universe's most chaotic entity has until Christmas Day to save the world.
He just has to finish his nap first.
Perfect for fans of Good Omens, The Dresden Files, and Terry Pratchett. If you like your eldritch horrors with a side of snark and a lot of heart, welcome to the holidays in Chicago.