I have a question about the pathfinder spell "feast of ashes". the question I have is how does it exactly work? I know the spell offers a save-roll on fortitude. but once the save failed is it possible to kill anyone with it? I would like to know what happens if a character (lets say Constitution 12 and 15 Hit Points) gets hit by the spell (the caster is level 3) and failed his fortitude save.
By a very strict interpretation of the RAW, yes, it can. The statement that makes it ambiguous is this sentence from the spell description: "The effects of this spell cannot inflict more nonlethal damage than the target has hit points". By a strict reading of the RAW, this sentence has no effect whatsoever: starvation can't do more non-lethal than your hp anyway, because at that point it stops doing non-lethal damage, and starts doing lethal instead. (from the starvation rules: "Characters that take an amount of nonlethal damage equal to their total hit points begin to take lethal damage instead.")
However, the context of this sentence seems to contraindicate it being a clarification, in favor of it being intended as an actual change. When a sentence with such context doesn't actually change anything, it's often an indication that the authors got sloppy with their phrasing or with interactions with other rules. In this case, I find it highly likely that the RAI was for the spell to bring a target to staggered (non-lethal equal to hp), and then just hold them there without further damage, not killing them.
Ultimately, it doesn't really matter which of these interpretations you use, except for how they fit into your campaign, so just pick whichever facilitates your plot, or if neither does, pick one at random. The spell works far too slow to be useful in combat, and is generally very unlikely to cause any balance issues, by either interpretation.
Yes, it will kill (slowly) according to the normal Pathfinder Starvation and Thirst rules: http://www.d20pfsrd.com/gamemastering/environment/environmental-rules#TOC-Starvation-and-Thirst
Feast of Ashes can not kill(directly). It says so in the description: The effects of this spell cannot inflict more nonlethal damage than the target has hit points.
Because of the way nonlethal damage works, the nonlethal damage must exceed the target's maximum hit points to begin to deal lethal damage, which the spell states that it doesn't do. What it does do is generally render the target unconscious at which point DEHYDRATION is what kills them.
What will happen in your example is on the failed save is that the target will begin taking 1d6 nonlethal damage a day for six days. If it takes a total of 15 points of nonlethal damage it will become unconscious and remain so for the remainder of the six days. If it has been unable to force itself to eat(the DC 12 Fort save) for over 3 days the nonlethal damage from the non-magically induced starvation may put it in danger of death. Of more concern to the target however is if it can not drink as that will do additional nonlethal damage.
Example(will be using 'average' damage of 4 on d6): Day1: Able to force eat/able to drink: Take 4 NL damage Day2: Able to force eat/drink: Take 4 NL damage Day 3: Able to force eat/drink: Take 4 NL damage Day 4: Fall unconscious: Take 3 NL damage take 1 day of 'real' starvation and dehydration Day 5: Remain unconscious: After 12 hours begin making CON checks(for maximum of 11 hours before DC becomes unreachable), then begin taking lethal damage at rate of 1d6/hour and be dead in no more than 12 hours Day 6: Death no later than 131 hours after spell is cast