Wasps! is there anything horrible they CAN’T do? Mother parasitoid wasps lay their eggs inside the bodies of other animals (in this case it’s caterpillars, but other species use tarantulas). When the wasp larva hatches, it hijacks the nervous system of the host and makes takes it somewhere dangerous (in the caterpillar’s case, to the end of a stick where it is stuck protecting a bunch of wasp pupas).
There are other examples of parasitic mind-control, such as zombie ant fungus and Toxoplasma gondii. The key is that parasites can make their hosts perform complex tasks.
So. What if there was a plant whose seeds parasitized nearby animals, forcing them to do work like weed and fertilize the plant before they sought out a nice patch of ground and died their, giving their bodies as nourishment for the new plant? Various species of parasitoid plants would evolve, warring against each other by means of puppetized animal proxies. Of course plants that took better care of their animal slaves (fed them, clothed them, provided shelter and intoxicants) would do better in the long run. The animals might be bred by the plants for strength or intelligence, and might spread to cover the cover the planet. The name of this plant, of course, is “wheat.”