Commentary
Class Skills
May want to remove references to exclusive skills. That's a 3.0ism.
Death Blow
Table refers to "Death Blow" at 3rd level. What is Death Blow?
Poison Affinity +3
Appears twice on the table, once at level 10 and once at level 12.
Improved Critical
There's no need to allow the assassin to choose another feat instead of Improved Critical if they already have it, in my opinion. Improved Critcal can be taken multiple times, after all.
Persistant Attacker
Not familiar with the Persistant Attacker Feat.
Spells
There's no description of the assassin's spellcasting ability, nor is there a spells known table. I addressed this in the post above.
Disemboweling Strike
Should be "Disembowelling". EDIT: Double checked this. Disemboweling is correct. Sorry about that.
There appears to be a formatting error in the description.
At 17th level, the assassin can sacrifice 4d6 of his sneak attack damage to, regardless whether or not he possesses the Weapon Focus feat.
To what (I assume it is the 1d4 Consitution damage)? What does dealing 1d4 Constitution damage have to do with possessing the Weapon Focus feat?
Why 1d4? It would be better to have a multiple of 2 for the effects of the ability.
Perhaps this would be better:
At 17th level, an assassin learns to deal horrific wounds to his victims. When making a sneak attack, the assassin can choose to forgo 4d6 point of extra sneak attack damage to deal 2 points of constitution damage to your opponent. You may make a disembowelling strike only once per day on any particular target.
You could have the Constitution damage be 4 if you want, but I wouldn't make it any higher. Another option would be to make allow the assassin to make the constitution damage variable, depending on the amount of dice sacrificed (1 damage per (2) dice, or something like that).
Death Comes to All
Minor typo
If an assassin can studies a group...
Does Death Comes to All have the same time restriction that Death ttack does?
20th level
Is kinda boring (like a rogue, I guess).
Perhaps a cool unique ability might be in order.