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): 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 wield at least three simultaneously.
For every two points in growth a viper tree possesses, it can wield an additional head at a time, to a maximum of eight (for those viper trees that are particularly 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 eight 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.
Fangs (s): each 'active' head on a viper tree can bite its would-be prey, inflicting its Strength score +2 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): 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 intensity 0 to intensity 10.
For every point of growth a viper tree possesses, add one point to its effective Strength score against ordinary sized opponents, while subtracting one point from its effective Agility score (to a minimum of zero (0)). Adjust this on the fly for foes of differing sizes.
Invulnerability to Cold, Corrosives and Poison Attacks (s): viper trees are incredibly hardy creatures, and possess full immunity to these three attack forms. This on top of their resistances to bashing damage, and bonuses for size (if any).
Poison (s): 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. Poisoning occurs as a contingent attack for each bite that does damage.
If the viper tree passes an easy Poison (strength) action against its opponent, it will paralyze him or her for the duration of the encounter - often enough time to consume them if they aren't helped. Even if the viper tree fails to eat its target, the victim will lose two points of Agility.
This ability score damage must be recovered normally. If the poison of a viper tree does not immediately paralyze a foe, it will still inflict a -1 Agility penalty for an aura duration with each bite, making it a good idea to flee the area - or to kill the attacking viper tree(s).
Resistance to Blunt Attacks (s): 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 intensity 4 (+1) protection against such harm.
Hindrances / Augmentations:
Susceptibility: viper trees are weak against the use of fire and heat attacks, what with their being part plant. Their supernatural defenses are ineffective against such assaults, and they take damage from fire and heat attacks as if their Strength score was zero (0).
Greed, with a secondary calling of Demolisher: the viper trees live to eat, and will gladly gorge upon anything they can get their mouths on. On the other hand, they have a noted hatred for baatezu, and will attack any such devil foolish enough to get within reach.
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.