Game of Thrones whitewalkers.
(This is all based on the tv series, the books show some things differently)
WHITEWALKERS
Whitewalkers are hostile fey, created as a weapon in an ancient war, who have since outlived the civilisation that created them, becoming a threat to everything that lives. When the power of winter grows, they come from the north, bringing along bitter cold, and armies of undead.
In the TV show a whitewalker is an acquired template - the whitewalkers prefer to transform human babies, bringing them up among themselves. The type changes to fey with a cold subtype.
Appearance: a whitewalker is a gaunt humanoid, with wrinkled, withered-looking skin, and piercing blue eyes. They tend to wear light armor and ride undead steeds.
Stats: Whitewalkers are much stronger and tougher than ordinary humans. They also enjoy a certain level of damage reduction, unless their weaknesses are exploited.
Abilities: Whitewalkers radiate an aura of deathly cold, which lowers temperature over a wide radius, and extinguishes fires within 5 feet. A marching army containing powerful whitewalkers can affect the weather to the point where not only is the cold physically dangerous, but those who freeze to death might rise as undead spontaneously.
High level whitewalkers might wield spellike abilities related to cold or necromancy.
Ice weapons: Whitewalkers wield weapons of magically reinforced ice. Despite their fragile appearance, the blades are deadly - they can shatter non-magical weapons as if they were glass, and an enemy slain by an ice weapon will soon rise as undead under the control of his killer.
Weaknesses: Due to their cold subtype, whitewalkers take double damage from fire. However, a whitewalker's cold aura nullifies that weakness - it will negate a fire attack, unless it is powerful enough to take the whitewalker down in one hit.
The whitewalkers also have a deadly weakness to two materials that represent the idea of fire in solid form, The first is 'dragon glass' (obsidian) - which is not only volcanic in origin, but was also the focus of the ritual that created them in the first place. The second is Valyrian steel, a very rare local alloy created through now lost alchemical process, probably involving dragon fire. Both these materials can withstand being shattered by whitewalkers' ice weapons, and can take them down in one hit, provided they draw blood (that is, they can still be stopped by armor, which might be a problem for stone weapons.)
WIGHTS
Wights are the undead created by the whitewalkers, either through necromancy, ice weapon kills, or rising spontaneously from the corpses touched by their cold. It is a common practice in the north to burn any people that die during winter.
Appearance: a wight is a walking corpse with striking blue eyes.
Abilities: wights are stronger and tougher that ordinary humans, stubbornly pushing forward despite horrible injuries (like with regular zombies, slashing weapons work best. ) Since internal damage is meaningless to them, they also take reduced falling damage. They gain climb speed, and improved grab - those that lack manufactured melee weapons will grab onto enemies to claw and bite. Unlike regular zombies, they retain all the speed they had with life, and can be relentless pursuers if ordered to.
Mindless: wights have no mind of their own, but instinctively obey nearby whitewalkers. If left to their own devices, they'll attack anything that moves except other whitewalker minions.
Weaknesses. Like their masters, wights are vulnerable to dragon glass and Valyrian steel. They also lack the whitewalkers' cold aura, leaving them with nothing to compensate for their cold subtype - fire attacks cause their flesh to melt like wax.
Decayed wights: the wights' undead state does not stop decomposition, though they are usually preserved by the frigid conditions their masters generate. Once a wight decays into a skeleton, his stats are severly reduced, and it becomes very brittle - he keeps most of his special features, including the reduced fall damage and climb speed, but gets reduced to one HD, and loses the damage reduction.