Here's an attempt:
Scourge of Keres (Su): Each ker wields a barbed whip, through which it can utilize several supernatural powers. In the hands of a non-ker, the scourge functions only as a masterwork whip/light flail. In the hands of a ker, it is treated as a +1 weapon and allows the ker to utilize additional powers.
First, the ker may make three attacks with the ker as a full attack action. It may make no other attacks in a round, regardless of the number of attacks it would normally be granted due to base attack bonus, feats, or spells (such as haste).
The first successful hit against a single target requires the victim to succeed on a DC X Will save or suffer bad luck for 3d10 minutes. This bad luck imparts a -1 to -4 luck penalty on attack rolls, saving throws, skill checks, and ability checks.
The second successful hit against a single target ages the victim 10d4 years unless it succeeds on a DC X Fortitude save. This effect is permanent, and can only be removed by a limited wish, wish, or miracle spell.
A third successful hit against a single target inflicts a supernatural disease upon the victim known as ker blight (Fortitude DC X, incubation period 1 minute; damage 1d6 Con and 1d6 Cha). Unlike normal diseases, ker blight continues until the victim reaches Constitution 0 (and dies) or is cured as described below.
Ker blight is a powerful curse, not a natural disease. A character attempting to cast any conjuration (healing) spell on a creature afflicted with ker blight must succeed on a DC 20 caster level check, or the spell has no effect on the afflicted character. To eliminate ker blight, the curse must first be broken with break enchantment or remove curse (requiring a DC 20 caster level check for either spell), after which a caster level check is no longer necessary to cast healing spells on the victim, and the ker blight can be magically cured as any normal disease.
An afflicted creature who dies of ker blight shrivels away into dust that blows away into nothing at the first wind.
The save DCs for all effects are Charisma-based.