Worldbuilding through the Rebuking Cleric

As Dan pointed out in his post, the cleric's Turn Undead ability says something about the default setting of D&D: there are undead here. Indeed, for this to be a core class core ability, there has to be a lot of them. Barrows teaming with draugr. Unquiet ghosts lingering near their graves in cemeteries. Abandoned castles ruled by a vampire lord and his brides.

But if we want to have a special theme for our campaign setting, one way we can broadcast that is to change the cleric's Turn Undead ability to focus on our main antagonists. 

Indeed, historically speaking, the sign of the cross was said to ward away almost anything evil, not just undead - devils and demons, werewolves, mermaids, storms, etc. A real swiss army knife. 

So. What does our D&D look like with different Turn Undeads? 

Turn worms

Saint Patrick drove the snakes from Ireland. He did this by turning worms and other crawling crawling things. 

At low levels, clerics can turn venomous snakes and serpents. At high levels, they can turn literal dragons. 

In such a setting, dragons plague the land. They kick dwarves out of their homes and sleep on their piles of gold. They crawl down into wells and poison the water. They bother the Smith of Wootton Major. Clerics go about, rebuking them in the name of Saint Patrick, and driving them continuously towards the wastes (as long as their luck holds).

Turn trolls

When Christianity came to the north of the world, the sign of the cross could drive away the trolls that haunted that land. (Of course, the trolls could smell a priest coming.)

A campaign setting framed in Norse mythology might give clerics the ability to rebuke trolls. In such a setting, clerics are adherents of a foreign religion, driving out the folk ways as much as they drive away trolls. 

Turn fairies

The ringing of church bells is one surefire way to drive away fairies of all sorts - banshees, dapplegrim, red caps, bugbears, hobby lanthorns, etc. 

A cleric in a Celtic themed campaign might drive away fairies instead of undead. These clerics keep the peace between human and fairy settlements, and serve as exorcists for mischief of all kinds - animate furniture? Call the priest. Baby crawling up the wall? Nope, that's a changeling, call the priest. Richard the Miller's Son came back with a wife with a hollow back? NOPE, CALL THE PRIEST. 

Stranger things

You can imagine not just defining a campaign setting through a cleric's mechanics but inverting and subverting them. 

A cleric might Turn Humans in an evil campaign, where they worship the dread lord Sauron. Here, clerics are sorcerers, sending forth their Black Breath to inspire fear. 

A cleric might Turn Apes in a far future campaign. They invoke the light of the Claw of the Conciliator, that most precious of jewels. An ape "destroyed" by the ability might join the cleric instead, offering their service.