In Dungeons and Dragons, some plants are “awakened”: they possess consciousness and mobility. And, of course, some awakened plants are evil and want to kill you. These are called “blights.”
Being plants, they derive nutrients from the soil, so they don’t need to kill to eat. They attack strictly out of spite.
Twig blights, found in forests and abandoned settlements in forested areas, are small and scrappy, with poor Strength but good Dexterity and Constitution. They don’t have high mobility, so they’ll compensate with numbers, growing in large patches. They also have proficiency in Stealth and the False Appearance feature, so ambushes are a likely tactic (insert Birnam Wood joke here). They have 60-foot blindsight and are vulnerable to fire.
Whenever a party of adventurers encounters a patch of twig blights, they’ll be inert, disguising themselves as ordinary desiccated shrubs. When victims come within reach, they’ll attack with surprise (give them advantage for hiding in plain sight, unless the PCs are watching out for them).
They’re cursed beings, not evolved creatures, so they don’t have much in the way of survival instinct; they attack until they’re destroyed. They’ll follow a foe who tries to retreat, but they won’t Dash after one. If their numbers are sufficient to surround an enemy, however, they will. And if attacked with fire, they’ll Dash away from the source.
Needle blights, found in evergreen forests, are human-size amalgamations of plant matter and needle-shaped leaves, which they can hurl like porcupine quills at opponents. Needle blights may be encountered in groups, but unlike twig blights, they aren’t always encountered in groups: they have sufficient Strength and Constitution to take on low-level opponents alone. The more suffused with evil influence an area is, however, the greater their numbers. They also aren’t ambushers—they have no Stealth skill. They’re not concerned with whether they’re seen, only with whether they’re approached (which they can detect out to a range of 60 feet, with blindsight).
Their Intelligence is animal at best, and while they aren’t slow, they also don’t move if they don’t have to. When a party of PCs comes within 30 feet of them, they attack with their needles; if one closes to within melee range, they attack with their claws. If the melee opponent then moves away, they make their opportunity attacks, but they may not bother to pursue, unless they’re trying to chase trespassers out of their part of the forest. They don’t flee when wounded and fight until they’re destroyed; they’re not afraid of fire.
Vine blights, found in forests and jungles, are vaguely humanoid-shaped masses of braided vines that can lash out and grab other creatures. They have some measure of Stealth—not a lot, but enough, when combined with their False Appearance, to blend into their surroundings in dim light—but are not dexterous, so while they do ambush victims, they wait until those victims get very close, then use Entangling Plants to restrain them. Finally, they engage restrained opponents in melee, grappling and squeezing them until they drop. They make opportunity attacks against retreating opponents and pursue to the best of their ability, although their movement speed is a less-than-impressive 10 feet. If they can catch a retreating opponent within the 15-foot radius of Entangling Plants, they do.
Vine blights, according to the Monster Manual flavor text, are connected to a “Gulthias tree,” a fiendish arboreal being for which there’s no listing in the MM. The online consensus appears to be that a Gulthias tree should be treated as an evil-aligned awakened tree. Awakened trees are huge brutes with a 10-foot reach; all they really do is smack opponents repeatedly with their branches and run away from fire. They’re not that interesting and only slightly challenging. If you’re in need of a more formidable opponent, you might rather go with an evil-aligned treant: these have many more hit points, are resistant to bludgeoning and piercing damage, and can also hurl rocks.
But given that the original Gulthias, according to the flavor text, was a vampire whose blood was so full of evil potency that it turned the wooden stake that skewered him into an undead sapling, it seems to me that something more is called for, such as a homebrew monster that’s part awakened tree/treant, part vampire—maybe that has the Regeneration and Charm abilities, does necrotic damage, or both. Something like this:
Huge plant, neutral evil
Armor Class 13 (natural armor)
Hit points 59 (7d12 + 14)
Speed 20 ft
Str 19 (+4) Dex 6 (−2) Con 15 (+2) Int 10 (0) Wis 10 (0) Cha 10 (0)
Damage Vulnerabilities fire
Damage Resistances bludgeoning, necrotic, piercing
Senses blindsight 120 ft (blind beyond this radius), passive Perception 10
Challenge 4 (1,100 hp)
Creeping Fog. Within 500 ft of the Gulthias tree, the forest is suffused with a thick miasma that blocks sunlight. The entire area is dimly lit and lightly obscured. A strong wind (at least 20 miles per hour) disperses it.
False Appearance. While the Gulthias tree remains motionless, it is indistinguishable from a normal tree.
Regeneration. The Gulthias tree regains 10 hp at the start of its turn if it has at least 1 hp and isn’t in sunlight. If the Gulthias tree takes radiant damage or damage from holy water, this trait doesn’t function at the start of the Gulthias tree’s next turn.
Multiattack. The Gulthias tree makes two attacks, only one of which may be an impale attack.
Slam. Melee Weapon Attack: +6 to hit, reach 10 ft, one target. Hit: 14 (3d6 + 4) bludgeoning damage. Instead of dealing damage, the Gulthias tree can grapple the target (escape DC 16).
Impale. Melee Weapon Attack: +6 to hit, reach 10 ft, one creature that is grappled by the Gulthias tree, incapacitated or restrained. Hit: 7 (1d6 + 4) piercing damage plus 10 (3d6) necrotic damage. The target’s hit point maximum is reduced by an amount equal to the necrotic damage taken, and the Gulthias tree regains hit points equal to that amount. The reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0. A humanoid slain in this way and then buried in the ground rises the following night as a vine blight under the Gulthias tree’s control.