BALANCE:
The class is evidently a variation of Rogue, with which it shares HD, BAB, ST, a similar skill list, Sneak Attack progression and Uncanny Dodge progression. For the "cost" of less skill points and no special rogueish abilities, it gets the Death Attack ability, some bonus vs poisons and a limited spellcasting ability, "the Wizard way" but with a short spell list, very few spells/day and only up to 4th level spells. The class seems very balanced on the overall.
FEATURES:
The most characteristic unique feature, the Death Attack, is clearly meant for non-combat purposes, that is it has its most usefulness in ambushes, which is very likely to be an Assassin's job. It is not necessarily a stunningly powerful ability, but after all even if the target makes the save there is still the Sneak Attack damage.
Poison abilities could have been expanded a little; they include just the 0% chance of accidentally poison himself while handling poison, and an improving bonus to ST. I wish there was a Craft(Poisonmaking) as exclusive skill, or at least as class-skill for Assassin which would be cross-class for all the core classes.
Sneak Attack is anyway probably the most useful feature. Although is it a replica of the Rogue ability, and therefore this PrCl does not introduce anything new about it, it is good to have another class with this feature.
Spells are IMHO designed almost perfectly for this PrCl. This is a character who is supposed to look forward for anything useful for assassination, being it weaponry, poisons, tactics or magic. The short progression (up to 4th level) and limited spells list show that
for the Assassin magic is just a mean, not an end or a general interest, and therefore he is not even remotely comparable with a full-time spellcaster. All the spells in the list have a reason to be useful for ambushing or killing and the choice of making it work like a Wizard is the best choice IMHO. A slightly longer list would have been nicer, since anyway he learns 2 spells/level + research, and a few more spells from PHB could have been included easily.
FLAVOR/RP:
It is usually more common to have non-evil PCs, and in fact there are definitely more Assassins in the NPCs ranks; I also think that the Death Attack is very difficult to use in combat, and PCs usually don't ambush but rather get ambushed.
The class emphasizes that the Assassin makes a profession out of it, hence the fact he belongs to a guild. Great flavor, but less appeal for a PC, who may be attracted by a PrCl's power rather than duties.
REQUIREMENTS:
Skills: very easy to reach by a PC with not too few skill points, and possibly taken by the very majority of Rogues around (with the exception of Disguise maybe). The ranks are set so that any class can reach them at most at 13th level, and it leaves the PrCl open to all, although not always to full extent.
Alignment: someone may disagree, but I think it is a fair req. As I said, the PrCl looks like it was written as a professional killer, who gets a target and proceed to kill him without questions.
RP: again, in line with the rest.
DESIGN:
I think it fits the stereotype perfectly, at least one of the killer's possible stereotypes.
I would have liked a longer weapon proficiency list, but the PHB doesn't have too many weapons geared toward stealthy murdering. As I said above, the abilities with poisons could have been expanded to something more complete.
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OVERALL: 9