Leechroots look like plant monsters. But actually, they're undead. And instead of creeping along like most plant creatures, they tend to burrow. In fact, they can even grapple creatures and then drag them underground, literally burying them alive. And their slashing roots cause bleed damage that resists even magical healing.
Bestiary 5 notes that leechroots “emerge from the remains of plants poisoned by the blood-drenched soils of war-torn forests.” This makes sense—battlefields tend to have coherent battle plans, clear rules of engagement, and hospital tents (however makeshift) nearby. On the other hand, fighting in the forest is close-up, dirty, and deadly, full of ambushes, quiet knifings, and men dying in the underbrush because their fellows couldn’t find them in time—all providing rich necromantic compost for an undead root.
Though it doesn't provide mechanics for it, B5 also suggests leechroots can turn dead plants into their spawn—a good excuse to slap an undead template onto your favorite plant creature. And while the exact mechanics are again not explicitly stated, leechroots in groups of four or more can form a sentient network known as a hivemind that serves primarily as a mutual-defense pact…and a promise of deadly, soil-covered retribution.
The secret vampire lords of Ornov quietly encourage the growth of leechroot on their lands, procuring cuttings from over the border in war-torn Haig. A leechroot’s presence easily explains any poorly buried, exsanguinated corpses, and a posse of men-at-arms leading a militia to burn out a leechroot infestation is a very visible—and popular—symbol of the princes’ dedication to their peasants’ well-being.
Vanaras and humans are engaged in a deadly battle over territory and logging rights. This long-running conflict has only festered with age—so much so that leechroots are now responsible for most of the casualties.
Wars of ideas can be as deadly as wars of arrows and blades. Adventurers visit an overgrown agora where philosophers used to debate in public. Here Galen abandoned the utilitarianism of his teachers, transforming his master’s model of the Cunning Mind to the brutal Culling Blade—his first step to becoming the genocidal monster known as the Hand in Bone. As the adventurers move throughout the complex, haunts offer glimpses of Galen’s development and fall—or rather, willful leap—from grace. Not only is surviving the haunts a challenge, but the agora is infested with leechroots, born from the hate engendered by Galen’s cruel, honeyed words.
—Pathfinder Bestiary 5 155
Pedro Coelho created the leechroot as an RPG Superstar entry. Check it and the judges’ comments out here.