Proper resolution of a Death Attack with a poisoned (injury) blade

Question

The player is making a Death Attack with a poisoned (injury) blade and wants the poison save to happen before the save from the Death Attack ability. As far as I can tell, these effects are simultaneously triggered by damage. His poison does Constitution damage. So he is hoping that a failed save on the poison will make the Death Attack save harder.

Death Attack (Ex): If an assassin studies his victim for 3 rounds and then makes a sneak attack with a melee weapon that successfully deals damage, the sneak attack has the additional effect of possibly either paralyzing or killing the target (assassin's choice). Studying the victim is a standard action. The death attack fails if the target detects the assassin or recognizes the assassin as an enemy (although the attack might still be a sneak attack if the target is denied his Dexterity bonus to his Armor Class or is flanked). If the victim of such a death attack fails a Fortitude save (DC 10 + the assassin's class level + the assassin's Int modifier) against the kill effect, she dies. If the saving throw fails against the paralysis effect, the victim is rendered helpless and unable to act for 1d6 rounds plus 1 round per level of the assassin. If the victim's saving throw succeeds, the attack is just a normal sneak attack. Once the assassin has completed the 3 rounds of study, he must make the death attack within the next 3 rounds.

Poison

Injury poisons are primarily contracted through the attacks of certain creatures and through weapons coated in the toxin. Injury poisons do not usually have an onset time and have a frequency of 1 round.

What is the proper order of resolution of these two effects?

Answer 1

The order is undefined by the rules

If the poison's effect are instantaneous, then the penalties are applied as soon as damage is taken and the attack is resolved. This is the case for most Injury poisons, such as the Black Adder Venom:

Type poison (injury); Save Fortitude DC 11

Frequency 1/round for 6 rounds

Effect 1d2 Con damage; Cure 1 save

If the poison has an Onset period, then it won't take effect immediatelly and will only apply it's penalties when that period has passed. Such as Wolfsbane (Onset 10 minutes), as described under Afflictions.

If you read Death Attack carefully, you will see that the conditions for it to work are making a successful attack (with sneak attack) that deals damage, so whatever effects of the attack have to be resolved before the character can use her Death Attack.

If an assassin studies his victim for 3 rounds and then makes a sneak attack with a melee weapon that successfully deals damage, the sneak attack has the additional effect of possibly either paralyzing or killing the target (assassin's choice).

This means that before you finish resolving the attack, you must roll all the damage (including sneak attack dice), with the possibility of your damage being reduced to 0 by defensive effects that are applied on damage, such as Damage Reduction, which could cancel the poison and the death attack ability.

However, since both the poison and the death attack are additional effects, the order of which must be applied first is undefined by the rules.

But usually the attacker chooses

There is a (unwritten) rule for Pathfinder that you resolve in the order that would be the most benefical to the creature. It is a carryover from 3.5's FAQ (has no working links left, but can be seen on this other answer, but never actually a written rule (or FAQ'ed) for Pathfinder, so GMs will have to fiat their own answer here.

For Pathfinder, as evidenced by the multiple threads on similar subjects, the community usually accepts that the attacker chooses the order of her own abilities, and the defender chooses the order of her own abilities. Other than that, the GM has to carefully read the abilities and use his own judgment there.

PS: Note that the answer given back then, for 3.5, is not valid for Pathfinder (vulnerability or energy resistance?), as we have a developer clarification in order of which to apply first, vulnerability then energy resistance, as seen on this other answer.