Large giant, chaotic evil
Armor Class 15 (natural armor)
Hit Points 103 (9d10 + 54)
Speed 30 ft., climb 30 ft.
Skills Perception +4
Damage Resistances poison
Senses darkvision 120 ft., passive Perception 14
Challenge 7 (2,900 XP)
Brute. A melee weapon deals one extra die of its damage when the troll hits with it (included in the attack).
Keen Smell. The troll has advantage on Wisdom (Perception) checks that rely on smell.
Regeneration. The troll regains 20 hit points at the start of its turn. If the troll takes acid or fire damage, this trait doesn't function at the start of the troll's next turn. The troll dies only if it starts its turn with 0 hit points and doesn't regenerate.
Relentless (Recharges after a Short or Long Rest). If the troll takes 10 damage or less that would reduce it to 0 hit points, it is reduced to 1 hit point instead.
Rampage. When the troll reduces a creature to 0 hit points with a melee attack on its turn, the troll can take a bonus action to move up to half its speed and make a bite attack.
Sunlight Sensitivity. While in sunlight, the troll has disadvantage on attack rolls, as well as on Wisdom (Perception) checks that rely on sight.
Multiattack. The troll makes three attacks: one with its bite and two with its claws. It may substitute the bite for a slam attack, but can't use the slam and claws against the same target.
Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 12 (2d6 + 5) piercing damage.
Claws. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 15 (3d6 + 5) slashing damage.
Slam. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 16 (2d12 + 5) bludgeoning damage.
The troll is a creature that is formidable in its own right. However, deep within the precipices of the Underdark, lives a diverged relative to them. Dumber, larger, and stronger, cave trolls are ferocious creatures that fight tooth and claw for a decent meal. As such, they're more aggressive and not keen on working with others, meaning those like the drow and duergar usually keep them in cages or gladiatorial pits and treat them as beasts.
Underground Monstrosity. Cave trolls are more formidable than their surface cousins in nearly every way. They are stronger, can see better in the dark, are resistant to poisons, and have better regenerative properties. They are even capable climbers, although instead of dexterous scaling they just slam their claws into the wall and pull themselves up. However, what they make up for in everything else they more than lack in brain power, even by the standards of trolls. These adaptations were likely due to the ferocity of the creatures in the Underdark.
Troll Abominations. Uncommonly, a surface troll may sustain a mutation due to its regenerative properties. However, due to the strength of their regeneration cave troll are far more mutable. It is common to see drow or deurgar arena masters decapitating or removing the limbs of cave trolls, or feeding them exotic creatures, in an attempt to create mutation. For example, a cave troll who loses a limb may grow two in its place, or if they lose a head they may grow two, or three, or one with a second face and brain, or if they eat an umber hulk they may get its powers of confusion, its digging skill, or some other trait.
However, due to the sheer mutability of their bodies, it is possible to undo the mutation if the cave troll loses the mutated part.
Variant: Loathsome Limbs
Some trolls have the following trait.
Loathsome Limbs. Whenever the troll takes at least 25 slashing damage at one time, roll a d20 to determine what else happens to it:
1-10: Nothing else happens.
11-14: One leg is severed from the troll if it has any legs left.
15-18: One arm is severed from the troll if it has any arms left.
19-20: The troll is decapitated, but the troll dies only if it can't regenerate. If it dies, so does the severed head.
If the troll finishes a short or long rest without reattaching a severed limb or head, the part regrows. At that point, the severed part dies. Until then, a severed part acts on the troll's initiative and has its own action and movement. A severed part has AC 13, 15 hit points, and the troll's Regeneration trait.
A severed leg is unable to attack and has a speed of 5 feet.
A severed arm has a speed of 5 feet and can make one claw attack on its turn, with disadvantage on the attack roll unless the troll can see the arm and its target. Each time the troll loses an arm, it loses a claw attack.
If its head is severed, the troll loses its bite attack and its body is blinded unless the head can see it. The severed head has a speed of 0 feet and the troll's Keen Smell trait. It can make a bite attack but only against a target in its space.
The troll's speed is halved if it's missing a leg. If it loses both legs, it falls prone. If it has both arms, it can crawl. With only one arm, it can still crawl, but its speed is halved. With no arms or legs, its speed is 0, and it can't benefit from bonuses to speed.