The Sylvan Incubus is a peculiar plant, looking like any other mostly barren tree, and can be found in most forests. Passersby would hardly know it's sinister nature as they just passed by. The tree doesn't look particularly ominous, nor does it ever move or shiver in a manner that would hint that it's a member of the necrofoenae. It has no crimson colored petals, or long prehensile vines that would indicate it preys on any creature. It's leaves are green, like any other tree, and they fall off in the fall and grow back in the spring.
What does make the Sylvan Incubus interesting is the curious fruit that lies about it's root stalks year round.
Any villager or peasant knows not to pick up strange fruit in the forest. And with good reason. The "fruit" at the foot of this tree is not actually a fruit at all - it's a symbiotic fungus that grows from the roots of the tree. The fungus assumes a ball-like shape, with the surface sheen of an apple or pear, though it is completely stalk-less, and tethered to the root by only a few thin strands of stringy material. Thus, it comes away from the ground with little effort. It weighs roughly the same as a common apple, and it smells absolutely delicious with the remarkably firm surface, and a strangely alluring smell. In fact, those who are in even the same forest as this remarkable fungus, can claim that they smell the succulent aroma from the moment they enter the forest - and are drawn, nay, pulled to find out just what is creating such a delicious smell.
Once they find it, of course they feel immediately starved and must sit down on the strangely comfortable roots of the tree above and enjoy a sumptuous treat on the fruit below.
Of course, this is somewhat blemished by the fact that the root-fungus is a highly potent, deadly neurotoxin.
Death is usually quick, after an epileptic onset of acute, violent seizures.
The fruit's job is done - the tree has a tasty snack of it's own now.
With surprising grace, the tree uproots itself, and uses one of it's uprooted stalks to sweep the body underneath the trunk. The trunk effectively sits back down over the body and begins to digest.
Dissections of the Sylvan Incubus lead us to believe that the tree opens it's stomach over the corpse, and lathers it with digestive enzymes which slowly dissolve the body into a nutrient soup, which is absorbed through the roots. There is evidence of a roughly maw-like structure on the underside of the trunk, which is commonly thought by Herbologists to be used to crack open bones to get at the marrow, though that is completely speculative.
What we do know is that the most common victims of the trick of the Sylvan Incubus are children, and city-folk who are caught unaware by the overpowering allure of the tree's "fruit." Forest paths are usually lined with warnings against eating strange fruit, since the presence of the Sylvan Incubus is completely unknown until it's far too late. And even when a hapless victim falls prey to it, he or she is unlikely to tell anyone else about it's whereabouts, or anything else, ever again.
Additional Ideas (1)
Sylvan Alp Tree
Alp trees are a member of the Necrofoenae family, and a very close cousin to the Sylvan Incubus Tree. Like the Sylvan Incubus, it produces a fungal growth that i much like a nondescript yet alluringly aromatic fruit. However, the fruit of the Sylvan Alp Tree kills in a very different manner.
Those who partake in it, fall deeply asleep into a catatonic state, filled with vivid, surreal dreams. These may be nightmarish in nature, or perhaps dreams of carnal lust. They are equally as likely to be dreams of penitence or glory - all depending on the persona of the victim.
Whereas the Sylvan Succubus Tree sweeps it's quarry underneath it's roots and digests it that way, the Alp Tree uses dexterous, vine like extensions to pick it up, and wrap tightly around it. From here, the tree produces micro-needles (about the size of a jellyfish's stinger) which slip into the pores of the prey. The sleeping prey is hoisted up into the thick branches of the tree to be digested, while it's still living; thus earning the tree it's vampiric nickname.
There are records of those who are strong enough of will to wake themselves from the dream the fungus induces - but only once the dreamer acknowledges that he is sleeping and his/her surroundings are in fact, immaterial. Not an easy task - considering that the poisonous fungus stimulates the persons mind in such a way that he is convinced that those around him are in fact real, by appealing to his personality through the use of familiar places and faces.
Those who wake have a slight struggle to writhe free of the prehensile grip of the Alp Tree, though the vines offer only token resistance. From here, the victim suffers no ill, except for a weeks worth of red, spotted sores covering all of his/her exposed flesh.
While the Incubus Tree fits into my world, I acknowledge after reading your comments that it's not entirely conducive to anything less than a party of UBER PCs. So this is my offering to all of those who want to use the basic idea of a carnivorous tree, in such a way that the PCs entire experience doesn't simply ride on a single save.