[Snake]

Dagger skill. Throw a barrage of daggers in a line, which will travel up to 3+Rank tiles away and inflict unresistable Piercing damage to the first enemy hit equal to Dagger Power + Rank. Can only be used once per turn.

This is kinda... 'eh'. First and foremost, Spellthieves -do not- use daggers, since they're locked to needing tomes to properly cast spells. Which makes it a huge problem itself.

It should be a general skill (i.e, not require a specific weapon type to be used) that deals protection-ignoring damage, sort of like this:

Flying Dagger
1M
10/9/8/7/6 FP

Spellthief skill. Throw a concealed dagger in a line, which will travel up to 3+Rank tiles away and inflict unresistable Pierce magic damage to the first enemy hit equal to Spellthief's LV + (Rank * 5), and inflicts Lingering Damage (LV = 50% of the damage) for 2 rounds. If already with Lingering Damage, instead increases LV by (Rank * 4) and duration by 2.

If the projectile hits an enemy who is not facing you, it ignores Evasion and reduces their Move by Rank until your next turn.

This skill is mostly for when a Spellthief is silenced and has no other combat options. It feels very thief-y for a person to have concealed throwing daggers hidden in them that they can have options when their backs are against the wall.

If the effect is too overbearing, just add something like Lingering Damage's LV will not increase higher than Spellthief LV * 3, or whatever. But it's only once per round anyway. As much as I wish you could throw a barrage of daggers one after another for a playstyle that involves using 1M, then 2M, then 4M to throw three daggers in a whole round.