We have something similar for blood warriors in the CC:
Want to shorten this?
I don't remember these critters at all.
I'll give it a go...
Necessary Slaughter #1 (Su): Undead dragon slayers must kill dragons to maintain their existence. If a slayer fails to kill dragons with Hit Dice totaling at least their own Hit Dice each week, the undead dragon slayer gains one positive level.
If a slayer kills a number of Hit Dice of dragons equal to its own total Hit Dice, it removes one positive level. If an undead dragon slayer currently has no positive levels, it gains 5 temporary hit points. These temporary hit points cannot exceed the slayer's full normal hit point total, and it gradually loses them at the rate of 1 hit point per day.
An undead dragon slayer with positive levels at least equal to its current level is instantly destroyed. After 24 hours, an undead dragon slayer must make a DC 18 Will save. If the saving throw succeeds, the positive level goes away with no harm to the creature. The afflicted creature makes a separate saving throw for each positive level it has gained. If the save fails, the positive level goes away, but the creature's level (or HD) is also reduced by one.
I don't like the permanent HD loss of the original and I think the "basic survival" nourishment level ought to be lower than the "gaining strength" nourishment level, so I'd change it to:
Necessary Slaughter #2 (Su): Undead dragon slayers must kill dragons to maintain their existence. If an undead dragon slayer fails to kill dragons with Hit Dice totaling at least half its own Hit Dice each week, the slayer gains one positive level.
If an undead dragon slayer kills a number of Hit Dice of dragons equal to its own total Hit Dice in a week, it removes one positive level. If a slayer currently has no positive levels, it gains 5 temporary hit points. These temporary hit points cannot exceed the slayer's full normal hit point total, and it gradually loses them at the rate of 1 hit point per day.
A slayer's positive levels from necessary slaughter can only be removed by slaying dragons. They never result in actual level loss, but if an undead dragon slayer ever gains positive levels equal to its current Hit Dice it is instantly destroyed.
I fancy allowing it to use its temp hit points to stave off the positive levels:
Necessary Slaughter #3 (Su): Undead dragon slayers must kill dragons to maintain their existence. If an undead dragon slayer fails to kill dragons with Hit Dice totaling at least half its own Hit Dice each week, the slayer gains one positive level. If the slayer has any temporary hit points from necessary slaughter (see next paragraph), it does not gain the positive level and takes 5 hit points of damage instead.
If an undead dragon slayer kills a number of Hit Dice of dragons equal to its own total Hit Dice in a week, it removes one positive level. If a slayer currently has no positive levels, it gains 5 temporary hit points. These temporary hit points cannot exceed the slayer's full normal hit point total, and it gradually loses them at the rate of 1 hit point per day.
A slayer's positive levels from necessary slaughter can only be removed by slaying dragons. They never result in actual level loss, but if an undead dragon slayer ever gains positive levels equal to its current Hit Dice it is instantly destroyed.
I like #3 the best.