Witchknifes as described by flavor text: unstoppable unnatural monsters who can control others with a single word.
Witchknifes as monsters: moderately annoying uglies who make a few party members do nothing for a bit, then die.
Witchknifes as PCs: hahaha no

Let me elaborate: the witchknife is a 9 RHD medium-sized monstrous humanoid, whose stats (+6 dexterity and charisma, +2 intelligence, +4 wisdom, -2 constitution) are uniquely suited to the kinds of classes that don't like HD. They have minimal fire resistance, sonic vulnerability, +2d6 innate sneak attack, and a few psionic abilities.

Said psionic abilities are at-will Command, Message, and on-self Silence, as well as 1/day Greater Command. Those aren't particularly impressive spells, especially at ECL 9. Against creatures immune to mind-affecting effects (or even things that just don't speak common), the witchknife's only real power comes from its (pretty weak) sneak attacks, and unfortunately for it many things are immune to both.

What to compare a witchknife to? If compared to a caster, all of those outclass it (1/day Greater Command is not exactly on par with 3/day Black Tentacles or Charm Monster). If compared to a rogue, it's got less than half the sneak attack damage and worse constitution, not to mention that the rogue is probably four levels into some prestige class already. Silence is a neat gimmick, but it's a gimmick that fails to impress at ECL 9.

-0 LA: remove half the HD and this may be more playable.