The origins of the viper trees are unknown. They seem to have some tanar'ri in them, but are not actually representative of those creatures. They also have properties of both reptiles and plants, all three fusing together to give them their curious set of abilities.
Additional Limbs / Heads: a viper tree initially has but three heads, but grows more once it lays down roots. It literally has dozens by the time it is fully grown, but can only control so many of them at a time. Of course, all viper trees can control at least three simultaneously.
For each increase in size factor a viper tree possesses, it can wield an additional head at a time, to a maximum of six (for those viper trees that are around 50 feet tall). It can inflict a bite attack with every head it can control each turn, which adds up fast on the larger trees.
Each 'branch' of a viper tree accounts for one of its Health points. One can chop off a dozen branches but the creature will still function (and attack) if it has any remaining. The easiest way to defeat a viper tree is to strike at its base, where all damage applies normally.
The creepy thing is that range is not a true protection from viper trees. If it cannot directly strike a foe, a viper tree may detach some of its 'branches' to reach foes at a distance. These heads will die after an hour, but can attack a foe normally once it reaches them.
Damage Resistance / Blunt Attacks: viper trees do not possess body armor, per se, but are highly resistant to Blunt Attack damage. Just like one would have difficulty cutting down a tree with a hammer, a viper tree possesses 2 CS of Damage Reduction against such harm.
Fangs: each head on a viper tree can bite its would-be prey, inflicting its Strength rank in Edged Attack damage with each strike. This isn't a lot of damage per bite, but a viper tree's fangs are the ideal delivery system for its dreadful, paralytic poison!
Growth: a viper tree begins at around six foot tall, but can reach over eight times this height with enough prey to feed upon. This means they may possess Growth (after a fashion) to represent their immense height, from anywhere to Shift 0 to Remarkable (30) rank.
When a viper tree reaches twelve feet tall, it gains a size factor of +1, while twenty four feet tall give it a size factor of +2, and finally, a height of forty eight feet tall gives it a size factor of +3. This makes the really large viper trees particularly dangerous.
For every +1 size factor a viper tree possesses in relation to its prey, it should receive a +1 CS to the damage it inflicts against them, though it hits at a -1 CS penalty. Similarly, smaller foes receive a -1 CS to damage inflicted, but gain a +1 CS to hit instead.
Invulnerability to Cold, Corrosives and Poison Attacks: viper trees are incredibly hardy creatures, and possess full immunity to these three attack forms. This on top of their other resistances to more mundane attack forms, and bonuses for size (if any).
Poison: the means by which it usually secures its prey, the viper tree is incredibly poisonous. Upon inflicting damage with its fangs, a viper tree will inject its poison into its would-be lunch. Those struck by this poison must pass an Endurance FEAT against Good (10) rank.
Failure to do so will render them paralyzed for 1d100 turns, and reduces one's Agility by -2 CS, ability score damage that can only heal with the passage of time. Even a successful resistance roll causes a temporary (1d100 turns) Agility reduction, in the form of uncontrollable shaking.
Allergy / Fire and Heat: while it is a hardy demonic creature, the viper tree is nonetheless highly vulnerable to fire and heat attacks. When subject to such forms of damage, they are considered +2 CS in rank before applied to the viper tree proper.
Viper trees are very dim, some being non-sentient and others barely so; this may or may not be a function of age. When they can do so, they speak only the tanar'ri language, such as it is. They rarely attack creatures smaller than themselves, unless they have a numerical advantage.
A viper tree looks like a beech tree from a distance, but up close one can see the scaly, reptilian bark of the creature. It looks like several wooden snakes that grew coiled together, each head extending off as its own 'branch' to writhe in the wind - or attack anything that gets close.
The viper tree is a strange hybrid of plant, reptile and demon - a sort of egg-laying tree. Their origins are lost to the mists of time, but said origins clearly included tanar'ri at one point, for the viper trees speak their language, shelter tanar'ri in battle, and always attack baatezu.
A viper tree lays an egg monthly, which is sheltered by the tree's bark until it hatches. At this point, it resembles a three-headed snake, which sets out to seek its prey. Once a viper tree hatchling manages this, it roots itself through the corpse of its victim, becoming sessile.
It then grows in size as fast as it can eat. While they start out only a few feet tall, and originally bear only three heads, a viper tree can grow up to fifty feet tall, and eventually possesses dozens of heads with which to consume prey - needing more to subsist at its greater size.
Vast groves of viper trees are common in the 45th to 47th layers of the Abyss - Graz'zt's realms. They also grow on the sites of the Blood War's greatest battles - at least until the baatezu do their level best to burn every last viper tree to the ground.
They occur elsewhere in the Abyss, most often when planted there by other Abyssal lords to serve as sentries or as pest removal. They can wander far and wide under their own power while seeking a suitable place to root, and can be found all over Carceri and the Gray Waste as well.