Tunnelrat
Tunnelrats are the elite scouts and patrollers of the Underdark, able to negotiate cramped tunnels and chambers, and operate as peerless guides, scouts, and spies in the dark tunnels of the underearth. They even hire out to patrol mines and infested sewers, if the price is right, and no small number find their unusual talents particularly suited to adventuring.
They wear thick, tough, tight clothing, and light armor, for mobility, and avoid loose straps, belts, cords, and anything else that might snag on projections when the Tunnelrat is traversing a narrow tunnel. Beards are tucked away and covered with a cloth, and thick leather pads are worn on knees and elbows as protection when crawling. A thick leather helmet with metal plates sewn inside is often worn for protection.
Tunnelrats combine courage with practical ruthlessness; in the cramped and dangerous confines of the Underdark, they cannot afford to take prisoners. This profession originally sprang up in the most desperate of the besieged dwarfholds, but has since spread as other relatively-small subterranean races, such as gnomes, svirfneblin, and duegar have produced similar specialists.
Rogues and Fighters find themselves drawn to this prestige class, as do many Cavers (dwarf Rangers). Few others find this prestige class particularly suited to their mindset.
Hit Die: d6.

Requirements:
To qualify to become a Tunnelrat, one must fulfill all the following criteria.
Base Attack Bonus: +3.
Feats: Alertness, Dodge, Mobility.
Skills: Hide 8 ranks, Move Silently 8 ranks.
Race: Any subterranean-dwelling race, not larger than a dwarf.

The Tunnelrat

Class
level	

Base
Attack Bonus	

Fort
Save	

Ref
Save	

Will
Save	

Special	
						
1st	+0	+0	+2	+0	Blind-Fight	
2nd	+1	+0	+3	+0	Sneak Attack +1d6, Uncanny Dodge (Dex Bonus)	
3rd	+2	+1	+3	+1	Stonecunning	
4th	+3	+1	+4	+1	Scuttle	
5th	+3	+1	+4	+1	Iron Lungs, Statue Stealth	
6th	+4	+2	+5	+2	Tunnel Fighter, Sneak Attack +2d6	
7th	+5	+2	+5	+2	Uncanny Dodge (Can't Be Flanked)	
8th	+6	+2	+6	+2	Rathead	
9th	+6	+3	+6	+3	Iron Heart	
10th	+7	+3	+7	+3	Sneak Attack +3d6	

Class Skills:
The Tunnelrat's class skills (and the key ability for each skill) are Balance (Dex), Craft (Int), Disable Device (Int), Heal (Wis), Hide (Dex), Intuit Direction (Wis), Jump (Str), Listen (Wis), Move Silently (Dex), Search (Int), Spot (Wis), Swim (Str), Tumble (Dex), Use Rope (Dex), and Wilderness Lore (Underground)(Wis). See Chapter 4: Skills in the Player's Handbook for skill descriptions.

Skill Points at Each Level: 6 + Int modifier.

Class Features:
All of the following are class features of the Tunnelrat prestige class.
Weapon and Armor Proficiency: Tunnelrats are proficient with the dagger, dart, hand crossbow, and short sword. Tunnelrats are proficient with light armor, but no shields. Note that armor check penalties for armor heavier than leather apply to the skills Balance, Climb, Escape Artist, Hide, Jump, Move Silently, Pick Pocket, and Tumble.
Blind-Fight: The Tunnelrat trains to use all their senses in the darkness of the subterranean world, and therefore gains the Blind-Fight feat.
Iron Heart: The monstrous terrors are the Underdark are many and terrible, but they are not the only fears the Tunnelrat must face: Daily, they risk becoming trapped in the tiniest of crevices and tunnels, drowning in labyrinthine submerged tunnels, suffocating in pockets of poisonous gas, falling to their dooms. Being a Tunnelrat requires great and unfaltering courage, and the most experienced members of this profession seem almost immune to the sensation of dread. The Tunnelrat adds a bonus of +4 to any saves they must make against fear, magical or mundane.
Iron Lungs: Pockets of stale air or gas, submerged tunnels, opponents with exceptionally keen hearing, all are threats the Tunnelrat must face. Because of this, they develop an astounding capacity to hold their breath, and can do so for a number of round equal to four times their Constitution score before they must begin to make Con checks to avoid drowning/suffocating. Use of this ability can add a +1 bonus to Hide or Move Silently checks when appropriate, at the DM's discretion.
Rathead: Because they must navigate and bypass some extremely difficult tunnels and crevices, Tunnelrats learn extraordinary abilities of contortion, and are able to squeeze through any opening large enough for their head to fit through. However, moving through such a small opening can be an extremely slow process, as the Tunnelrat gradually inches through, dislocating and rearranging limbs as he does so.
Scuttle: Stealth is one of the Tunnelrat's primary survival traits, and they learn to make use of every bit of available cover, while remaining as mobile as possible. They can crawl on their hands on knees — thereby making better use of what little cover may be available — at their full walking speed.
Sneak Attack: At 3rd level the Tunnelrat learns to be ruthless when facing opponents, and take every advantage of ambush. If the Tunnelrat can catch an opponent when she is unable to defend herself effectively from his attack, he can strike a vital spot for extra damage. Basically, any time the Tunnelrat's target would be denied her Dexterity bonus to AC (whether she actually has a Dexterity bonus or not), the Tunnelrat's attack deals +1d6 points of damage. This extra damage increases by +1d6 points at 6th and 10th level. Should the Tunnelrat score a critical hit with a sneak attack, this extra damage is not multiplied. It takes precision and penetration to hit a vital spot, so ranged attacks can only count as sneak attacks if the target is 30 feet away or less.
With a sap or unarmed strike, the Tunnelrat can make a sneak attack that deals subdual damage instead of normal damage. He cannot use a weapon that deals normal damage to deal subdual damage in a sneak attack, not even with the usual -4 penalty, because he must make optimal use of his weapon in order to execute the sneak attack.
A Tunnelrat can only sneak attack living creatures, with discernible anatomies—undead, constructs, oozes, and incorporeal creatures lack vital areas to attack. Additionally, any creature immune to critical hits is similarly immune to sneak attacks. Also, the Tunnelrat must be able to see the target clearly enough to pick out a vital spot and must be able to reach a vital spot. The Tunnelrat cannot sneak attack while striking at a creature with concealment or by striking the limbs of a creature whose  vitals are beyond reach. If a Tunnelrat gets a sneak attack bonus from another source (such as Rogue levels), the bonuses to damage stack.
Statue Stealth: The Tunnelrat learns the value of endless patience, and the technique of remaining utterly immobile for many hours. He can lie perfectly still for a number of hours equal to his Constitution score, and may gain a +2 bonus to Hide checks if appropriate, at the DM's discretion. This ability, when coupled with Iron Lungs, enables the Tunnelrat to feign death with alarming accuracy.
Stonecunning: The Tunnelrat quickly becomes intimately familiar with the subterranean world. They get a +2 bonus on checks to notice unusual stonework, such as sliding walls, stonework traps, new construction, unsafe stone surfaces, shaky stone ceilings, and the like. He can also notice unusual stonework, and intuit depth, just like a dwarf. If the Tunnelrat already has Stonecunning, his check bonus when using this ability increases to +3.
Tunnel Fighter: The Tunnelrat learns to fight effectively in even the most restrictive of spaces, and suffers no attack or AC penalties in areas as small as 2 feet across, when fighting unarmed, or using a dagger or shortsword. He can even fling darts for full effect with only 1 foot of throwing space in front of him or to one side.
Uncanny Dodge: Starting at 2nd level, the Tunnelrat gains the extraordinary ability to react to danger before her senses would normally allow her to do so. At 2nd level and above, she retains her Dexterity bonus to AC (if any) regardless of being caught flat-footed or being struck by an invisible attacker. She still loses her Dexterity bonus to AC if immobilized. At 7th level, the Tunnelrat can no longer be flanked, as she can react to opponents on opposite sides of her as easily as she can react to a single attacker. This defense denies other characters the ability to use flank attacks to sneak attack her. The exception to this defense is that a Rogue at least four levels higher than the character can flank her (and thus sneak attack her).
The character's Tunnelrat levels are cumulative with her levels in any other class that has the uncanny dodge ability for the purposes of determining the character's total uncanny dodge.