Assassinate (Ex): A ninja with this master trick can kill foes that are unable to defend themselves. To attempt to assassinate a target, the ninja must first study her target for 1 round as a standard action. On the following round, if the ninja makes a sneak attack against the target and the target is denied its Dexterity bonus to AC, the sneak attack has the additional effect of possibly killing the target. This attempt automatically fails if the target recognizes the ninja as an enemy. If the sneak attack is successful and the target of this attack fails a Fortitude save, it dies. The DC of this save is equal to 10 + 1/2 the ninja’s level + the ninja’s Charisma modifier. If the save is successful, the target still takes the sneak attack damage as normal, but it is immune to that ninja’s assassinate ability for 1 day.
Breath of life: This spell cures 5d8 points of damage + 1 point per caster level (maximum +25).
Unlike other spells that heal damage, breath of life can bring recently slain creatures back to life. If cast upon a creature that has died within 1 round, apply the healing from this spell to the creature. If the healed creature's hit point total is at a negative amount less than its Constitution score, it comes back to life and stabilizes at its new hit point total. If the creature's hit point total is at a negative amount equal to or greater than its Constitution score, the creature remains dead. Creatures brought back to life through breath of life gain a temporary negative level that lasts for 1 day.
Creatures slain by death effects cannot be saved by breath of life.
Like cure spells, breath of life deals damage to undead creatures rather than curing them, and cannot bring them back to life.
Is there anything in RAW that I'm missing regarding a Ninja's Assassinate ability and how it would work with breath of life? It doesn't explicitly state that it is a death effect, which is good or bad, depending on spells like death ward or breath of life. If Assassinate is not a death effect and a creature can be brought back by breath of life then how would the creature's hit points be determined?
I'd be inclined to roll damage normally and set that as the negative hit points, but this has no basis other than simplicity.
In case it matters, a dead character, no matter how he died, has hit points equal to or less than his negative Constitution score.
In Death Attacks it does state that Deathward helps protect you.
In this case, SKR's famous, "If it looks like a duck, quacks like a duck, then it probably is a duck" comes into play.
So Breath of Life would restore the target fairly easily.