People who write background stuff in fantasy video games have the most fun.
Thanks to Twitter user FioraAeterna for digging up this codex from Dragon Age: Inquisition. The story of "Plants vs Corpses" concerns a "peculiar" story of reanimated corpses ranging in Ferenden, in the land of Thedas. Only a magician with the ability to turn plants into warriors can halt their deathly advance.
Plants vs Zombies games are, of course, published by Electronic Arts, which is also the company behind Dragon Age. It's probably worth repeating that we really like Dragon Age: Inquisition.