Bladevine

This dark creeper thrives in almost any conditions, grows several feet a day, can bury a small building in a week, and has few grazers to check it.

One plant may have 2 to 20 vines rising from a shared root, twisted into knots so tangled nobody can tell how many must be cut to get through. Usually every vine on one surface belongs to one plant, though a castle or cliffside might host dozens of patches whose edges mingle.

Its small, heart-shaped leaves are almost black, clustered on short wiry sprigs near the stem. Their serrated edges are harmless. The danger is the stem, a three-sided shaft whose raised ridges are iron-hard and sword-keen, ready to split a careless forearm to the bone.

The mindless, motionless vine is easy to avoid, yet someone who falls in without good armor may bleed to death before escaping. It wounds in three ways.

Handling it means wading through carefully, reaching in for something, or climbing a vine-covered wall. Each round of this requires a save vs. death magic or a Dexterity check (DM's choice), and failure deals 1d3 plus the character's base Armor Class.

Cutting it is nearly as risky, since the twisted vines are under tension and snap back when severed. Every vine cut with a hand weapon or tool makes one attack against the cutter's normal AC, dealing 1d4 plus base AC on a hit with no save, and anyone caught in the patch is hurt too.

Falling in deals 1d6 plus base AC automatically, with no save or attack roll, though a save to avoid the fall may apply. Someone in the patch takes no more damage while still, but every round of moving or struggling repeats the full falling damage, and climbing out usually takes 1 to 3 rounds.

Base AC is armor alone, ignoring Dexterity, shields and magic that does not cover the whole body or works by misdirection. Blur is useless, but an armor spell helps, and protection from a ring or cloak counts, while boots of striding and springing or bracers of defense do not.

Only Type S weapons hurt a vine, which has AC 5 and 5 hp. Cutting half the vines opens a path or frees a trapped ally, while cutting all of them clears the plant, which regrows within days unless its roots are dug up and destroyed. Bladevine resists fire. Ordinary flames just burn off the leaves and harden the stems; only magical fire truly damages them.

It grows nearly everywhere, but especially in the great planar city and in the more hellish parts of the neutral and lower planes. A patch makes a fine sentry, and a few cuttings from a healthy vine, plus some training, will cover a wall or seal a doorway.

Humanoids cannot eat it, but some hardy beasts can chew and digest it, so it serves as fodder for specialized livestock. One gate-town's vintners make a dark wine from the leaves and guard the method closely.

Dried cuttings burn well; oiled ones become razor-edged ropes, whips and cords. Binding a prisoner with bladevine inflicts handling damage once, and again each time the captive struggles. A bladevine garrote adds that damage to a normal garrote's.

No one knows its true origin or how it thrives anywhere, but it has a hellish look, and most suspect it came to the planar city from some toxic jungle of the Abyss. Merchants hoping for cheap security have carried cuttings into the neutral planes, only to learn how virulent it is. One gate-town reportedly had a smuggler drawn and quartered for sneaking in cuttings right after the town had finished clearing the stuff out.