A Grimthorn is a cathedral-sized carnivorous plant, found exclusively in the upper canopies of vast jungles. The mass of trunk-thick black aerial roots anchored in the upper canopy allow the plant to position its singular enormous, thorn-rimmed flower-maw near the jungle floor. The ambusher preferes arboreal man-made constructions, like treehouses, to lay ambush.
I recently had an opportunity to submit a monster to Kobold Press' Tome of Beasts 3 through their Kickstarter. My submission didn't get chosen, which means I can freely share it with you myself.
Be sure to check out the treehouse map, as this is a perfect monster for it.
Included are VTT pieces and monster stats for D&D 5e.