Toothless

Toothless (2010) is a historical fantasy/horror novel by J.P. Moore. From the back cover:

FRANCE, 1180 A.D.
Judgment Has Come
An ancient evil leads a rampaging army of demons and undead warriors across the countryside. Martin, a failed Templar, is slain on the field of battle only to be reanimated in service to the very evil he hoped to destroy. The Black Yew, the dark force that controls the undead army, considers him a gifted minion.
But life is not done with him yet.

The book features zombies, the Knights Templar, zombie Templars, visionaries, Irish monks, werewolves, demons, plague, and the Black Yew, an eldritch abomination in the form of a gigantic evil tree. It's also notable for its use of an undead, speechless protagonist, and for its medieval take on the zombie apocalypse.

Toothless originally appeared in 2009 as podfic, read by the author, and the whole audiobook is available here. Read the first three chapters here.

This book is not to be confused with the 1997 film starring Kirstie Alley, or with the character from How to Train Your Dragon.

Tropes used in Toothless include:

And I Must Scream: Toothless, in an unusually literal way.
Attack! Attack! Attack!: the Black Yew's preferred method of combat.
Awesome McCoolname: Spoiler.
Baleful Polymorph: The werewolves' transformation only seems to work one way.
BFS: The angel greatsword.
Blessed with Suck: Lil, the deformed teenage seer.
A Boy and His X: A Girl And Her Zombie.
Dem Bones: Longinus and the other captains.
Eldritch Abomination: the Black Yew falls somewhere between this and Cosmic Horror.
Fail O'Suckyname: the cart demon gives the dead men demeaning new names when they're revived as zombies.
Fan Disservice: One character rips another's clothes off. It is... the polar opposite of sexy.
Fluffy the Terrible: shades of this with the ironically-named Brother Francis.
Freudian Trio: With Toothless as ego, Breakneck as id and Curdle as superego.
Fully-Embraced Fiend: Spoiler; early on, Toothless himself.
Gorn: Lots of detailed descriptions of battle and messy slaughter.
Heterosexual Life Partners: Breakneck and Curdle.
Ho Yay: given that most of the characters are male, inevitable.
Kid with the Leash: Lil with Toothless.
The Knights Templar
Large Ham: Lots of characters in the audio version, in the best possible way. The voice of the Black Yew, for example. "FALLEN. ARRIIIIISE!"
Morality Pet: Lil is this for Toothless.
Not Using the Zed Word
Our Werewolves Are Different
Our Zombies Are Different: There are "shamblers" (mindless, lurching zombies) and "sentients" (fast zombies who can still think like humans). No one eats brains, but the sentients feed on the life force of humans whom they kill.
The Power of Friendship: a recurring theme, as is...
The Power of Love
The Speechless: Toothless.
Those Two Bad Guys: Breakneck and Curdle have elements of this.
Waif Prophet: Lil has elements of this.
We ARE Struggling Together!: much of humanity is too busy fighting other humans to band together against the Yew.
Zen Survivor: Cedric.
Zombie Apocalypse: obviously.