Knives

Knives have the fastest and cheapest swings in the game, and they are the only weapons that hit an unaware enemy for 6 times damage, where everything else manages 3.

Combat

Pros

6x damage against an unalerted target, double what any other weapon gets from the same opening. A knife build is really a Sneaking build with a finisher attached.
A swing costs 4 to 14 stamina, the cheapest in the game, and the 3-hit combo takes 1.74 seconds, the fastest of the melee classes.
The primary attack does not move you at all, so you stay exactly where you chose to stand. Nothing else lets you hold a position that precisely.
The secondary lunges 5 metres forward for 3x damage and 4x knockback. That is how the class closes the reach gap instead of living with it.

Cons

1.8 metres of reach, the shortest in the game. A polearm hits from 3.2, and a Fuling does not wait for you to close in.
Slash and pierce together are the worst pairing in the game for finding weaknesses. Nothing is weak to slash at all, and pierce is the most resisted type there is.
A low ceiling: the Nord Dagger stops at 150, and even the Frostfire Dagger only reaches 222. Once the opening backstab is spent, you are fighting with the weakest numbers of any melee class.
2 block armour is effectively none, so the build is committed to a shield or to never getting hit.

Builds

The opener is the build. Level Sneaking first, because it works off the light where you stand: at skill 0 an enemy's view range is halved in full darkness, and at 100 it drops to a fifth. Land one backstab at 6x, and from there you are in a normal fight with a small weapon.

Wear dark armour and carry no torch and no glowing sword. The Mistwalker and the Jotun Bane both raise your visibility by about a fifth just by being drawn, which undoes the skill you have been levelling.

Then add a shield. A knife contributes nothing on defence, so the off-hand is your whole guard. The shield replaces the knife's block values entirely, so take a buckler for parrying.

The secondary is your escape as well as your approach. A 5 metre lunge for 3x damage and 4x knockback is also 5 metres you can put between yourself and a pack.

Knives are made at the Forge, the Black forge, the Workbench and the Frost Foundry.

The skill is Knives, and it levels by use rather than by choice.

Best Knives

Build	Name	Stats
Hardest hitter	Frostfire Dagger	222 damage across all types
Cheapest to swing	Flint Knife	4 stamina a swing
Best guard	Butcher Knife	2 block armour
Longest lasting	Frostfire Dagger	400 durability

Notes

A knife's 6x sneak attack bonus only applies to an enemy that has not noticed the player yet. Hitting an enemy from behind while it fights someone else does not count.
The primary combo hits twice for normal damage and then once for double damage, and the whole combo takes 1.74 seconds.
Its primary attack has the weakest knockback in the game, at 10, so enemies stay in reach for the whole combo instead of being pushed away.