Phytocarnivory and Arcane Accumulation

Magic tainted by corruption settles most easily into plants, and to some degree or another always makes them carnivorous. At the lowest ambient arcane concentrations, plants act as one may expect of a normal plant. But when a living creature draws near, the leaves of the plant slowly bend toward it. Sticky saps begin to ooze from any green surface, even from plants that don't normally produce saps. On contact with prey, the leaves latch on.
The degree of carnivory and mobility imbued upon the plants increases as energy gathers. When magic accumulation is low, it is not a strong connection, and easy to escape the grip of the plants. In the next stages, the leaves of plants send small, nearly microscopic, tendrils into the flesh of the creature it has latched on to. Once embedded, these tendrils secure the grip of the plant and then rapidly drink in the blood of its victim.
The next stage of accumulation sees plants becoming more mobile. Plants will pull away from other areas to grab prey. Spontaneous rapid growth may also occur, taking several forms: swells of grass may form waves that attempt to roll over prey, pillars or "fingers" of sudden growth may reach up to grab creatures that are off the ground, plants may totally change shape and structure to form anatomies that allow for hunting. There are documented instances of flowers forming harpoons, or spontaneously developing the ability to launch acid, for example.
If enough energy is present, plants can warp and wrap together enough to become mobile creatures, retaining their hunger. Shambling mounds are extremely common dangers on the plains, reminders of the dangers of casting magic or having people grouped too closely together.

Triggers

There are two main attractants that pull in the corrupt magic that causes phytocarnivory: use of magic (generally through the casting of spells), and the presence of animals, especially in groups. These conditions draw in nearby energy like a magnet collecting iron filings.

Biomagnification

The tainted energy collected in the plants takes a long time to dissipate. Though the higher concentrations usually drop to lower tiers of accumulation over the course of a few days, the energy almost never fully leaves a living plant. As plants are eaten by herbivores, that magic sticks more strongly to animals that eat the plants than the vegetation. Arcane accumulation condenses inside the herbivores, and when the herbivores are consumed by predators or scavengers, that condensed magic builds in their gut, and so on as they are predated by larger predators. As this energy builds, it can cause odd physical and magical mutations. Through the effects of biomagnification, larger carnivores are most likely to be afflicted by the taint of the arcane, and frequently have mutations and magical abilities that are not naturally present.

Manifestation

When arcane accumulation is high enough, the area will appear to be under a haze. Magic in the area is technically not visible to the eye, but it gives the impression of being a grainy black filter over the area.

Localization

In areas where there is little plant growth, such as deserts, the effects of arcane accumulation are much more concentrated in the available plant growth. Without plants to settle in to, the corrupted energy keeps drifting until it can be absorbed. This leads to oases becoming a death trap, though animals and people still need to risk it for access to water.