Let's go with Str +4, Dex +4, Int +6, Wis +4, Cha +4.
When it becomes undead, a king-wight gains many special abilities. A successful attack can drain two life levels from a victim, as per a vampire. Any victim completely drained of life points by the kingwight becomes a full-strength wight under the control of the king-wight.
Create Spawn (Su): Any humanoid slain by a king-wight becomes a wight in 1d4 rounds. Spawn are under the command of the king-wight that created them and remain enslaved until its death. They do not possess any of the abilities they had in life.
Energy Drain (Su): Living creatures hit by a king-wight’s slam attack gain two negative levels. The DC is X for the Fortitude save to remove a negative level. The save DC is Charisma-based. For each such negative level bestowed, the king-wight gains 5 temporary hit points.
A king-wight also has the ability to cast spectral force and confusion spells, one spell per round, without limit. It can teleport once per day, but only to or from its barrow home.
per the conversion guide, spectral force = major image.
So confusion and spectral force as at-will SLAs? Greater teleport 1/day?
When the king-wight is destroyed, the action causes an earthquake (as per the clerical spell, at the 14th-level of effect), centered on the king-wight's body, in 4-16 rounds. Since a king-wight is often encountered in its underground barrow, such an earthquake can be especially deadly.
This sounds like a death throes ability.
A king-wight is so powerful that any individual of a level lower than the kingwight must make a saving throw vs. spells or flee in panic from fear.
Fear aura or frightful presence?
The following spells or attack forms have no effect on a king-wight: charm, sleep, enfeeblement, polymorph, cold, electricity, insanity, and death magic. A raise dead spell turns the king-wight into a normal 12th-level fighter unless a saving throw vs. spells is made.
Most of that is handled by the undead type, but we can add in immunity to cold and electricity.
A cleric attempting to turn a king-wight should use the "special" column. A king wight can be harmed only by magical weapons.
We should also retain this...
Skills: Wights have a +8 racial bonus on Move Silently checks.