In light of recent reading into Pluto, I noticed that it has the unusual quality of being reciprocally tidally locked to its moon Charon. I have taken this idea and am going to use it in a fantasy campaign wherein an artifact triggers the planet's rotation to slow down to match that of its moon's orbit. I plan on using the earth/moon for rotation speeds until eventually the day on the planet is 28 normal days, and the planet is tidally locked to the moon. In this magical world, this process will take several weeks.
Edit: Through some perusing I have found that there are a multitude of questions asking how climates/weather/etc. will be affected by an earth-like planet becoming tidally locked or having a slowed or stopped rotation on the order of the one I am proposing. In light of this I will narrow the question:
Assume that effects on the natural, physical world are somehow made negligible (including effects on plants and animals, ocean and air currents, etc). What effects could this "slow down" have on medieval level cultures? For example, half of the planet will no longer be able to see the moon, while it will always be visible in the sky for other peoples.