Hit Die: d6.

Class Skills: The assassin’s class skills (and the key ability for each skill) are Balance (Dex), Bluff (Cha), Climb (Str), Craft (Int), Decipher Script (Int, exclusive skill), Diplomacy (Cha), Disable Device (Int), Disguise (Cha), Escape Artist (Dex), Forgery (Int), Gather Information (Cha), Hide (Dex), Innuendo (Wis), Intimidate (Cha), Intuit Direction (Wis), Jump (Str), Listen (Wis), Move Silently (Dex), Open Lock (Dex), Pick Pocket (Dex), Read Lips (Int, exclusive skill), Search (Int), Sense Motive (Wis), Spot (Wis), Swim (Str), Tumble (Dex), Use Magic Device (Cha, exclusive skill), and Use Rope (Dex).

Weapon and Armor Proficiency: Assassins are proficient with the crossbow (hand, light, or heavy), dagger (any type), dart, rapier, sap, shortbow (nor­mal and composite), and short sword. Assassins are proficient with light armor but not with shields.

1 +0 +0 +2 +0 Sneak attack +1d6, Death Attack, Contract
2 +1 +0 +3 +0 +1 save vs. poison, Uncanny Dodge
3 +2 +1 +3 +1 Sneak Attack +2d6 
4 +3 +1 +4 +1 +2 save vs. poison
5 +3 +1 +4 +1 Sneak attack +3d6, Uncanny Dodge (can't be flanked)
6 +4 +2 +5 +2 +3 save vs. poison
7 +5 +2 +5 +2 Sneak attack +4d6 
8 +6/+1 +2 +6 +2 +4 save vs. poison
9 +6/+1 +3 +6 +3 Sneak attack +5d6 
10 +7/+2 +3 +7 +3 +5 save vs. poison, Uncanny Dodge (+1 vs. traps)
11 +8/+3 +3 +7 +3 Sneak attack +6d6
12 +9/+4 +4 +8 +4 +6 save vs. poison
13 +9/+4 +4 +8 +4 Sneak Attack +7d6 
14 +10/+5 +4 +9 +4 +7 save vs. poison
15 +11/+6/+1 +5 +9 +5 Sneak attack +8d6, Uncanny Dodge (+2 vs. traps.)
16 +12/+7/+2 +5 +10 +5 +8 save vs. poison
17 +12/+7/+2 +5 +10 +5 Sneak attack +9d6 
18 +13/+8/+3 +6 +11 +6 +9 save vs. poison
19 +14/+9/+4 +6 +11 +6 Sneak attack +10d6 
20 +15/+10/+5 +6 +12 +6 +10 save vs. poison, Uncanny Dodge (+3 vs. traps)



Spells Per Day

1st 2nd 3rd 4th
1 0 -- -- --
2 1 -- -- --
3 1 0 -- --
4 1 1 -- --
5 1 1 0 --
6 1 1 1 --
7 2 1 1 0
8 2 1 1 1
9 2 2 1 1
10 2 2 2 1

Sneak Attack: Any time the assassin’s target would be denied her Dexterity bonus to AC (whether she actually has a Dexterity bonus or not), the assassin’s attack deals +1d6 points of damage. This extra damage increases by +1d6 points every other level (+2d6 at 3rd level, +3d6 at 5th level, and so on). Should the assassin score a critical hit with a sneak attack, this extra damage is not multiplied.

It takes precision and penetration to hit a vital spot, so ranged attacks can only count as sneak attacks if the target is 30 feet away or less.

With a sap or an unarmed strike, the assassin can make a sneak attack that deals subdual damage instead of normal damage. He cannot use a weapon that deals normal damage to deal subdual damage in a sneak attack, not even with the usual –4 penalty, because he must make optimal use of his weapon in order to execute the sneak attack.

An assassin can only sneak attack living creatures with discernible anatomies—undead, constructs, oozes, plants, and incorporeal creatures lack vital areas to attack. Additionally, any creature immune to critical hits is similarly immune to sneak attacks. Also, the assassin must also be able to see the target well enough to pick out a vital spot and must be able to reach a vital spot. The assassin cannot sneak attack while striking at a creature with concealment or by striking the limbs of a creature whose vitals are beyond reach.

If an assassin gets a sneak attack bonus from another source (such as rogue levels), the bonuses to damage stack.

Death Attack: If the 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). While studying the victim, the assassin can undertake other actions so long as his attention stays focused on the target and the target does not detect the assassin or recognize the assassin as an enemy. If the victim of such an attack fails her Fortitude saving throw (DC 10 + the assassin’s class level + the assassin’s Intelligence modifier) against the kill effect, she dies. If the saving throw fails against the paralysis effect, the victim’s mind and body become enervated, rendering her completely 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. If a death attack is attempted and fails (the victim makes her save) or if the assassin does not launch the attack within 3 rounds of completing the study, 3 new rounds of study are required before he can attempt another death attack.

Poison Use: Assassins are trained in the use of poison and never risk accidentally poisoning themselves when applying poison to a blade.

Spells: Beginning at 1st level, an assassin gains the ability to cast a small number of arcane spells. To cast a spell, the assassin must have an Intelligence score of at least 10 + the spell’s level, so an assassin with an Intelligence of 10 or lower cannot cast these spells. Assassin bonus spells are based on Intelligence, and saving throws against these spells have a DC of 10 + spell level + the assassin’s Intelligence modifier (if any). When the assassin gets 0 spells of a given level, such as 0 1st-level spells at 1st level, the assassin gets only bonus spells. An assassin without a bonus spell for that level cannot yet cast a spell of that level. The assassin’s spell list appears below. An assassin casts his spells like a runeweaver and is one of the few groups who have discovered the secret of casting runes.

Saving Throw Bonus vs. Poison: Assassins train with poisons of all types and slowly grow more and more resistant to their effects. This is reflected by a natural saving throw bonus to all poisons gained at 2nd level that increases by +1 for every two levels the assassin gains (+1 at 2nd level, +2 at 4th level, +3 at 6th level, and so on).

Contract:  Assassin contracts come through the guild.  The contract amount is never known by the assassins but they are well compensated.  Assassins are matched to the contracts according to their skills.  Sometimes, a contract is so difficult that it requires multiple assassins to complete the task.

It is not unknown for a contract to require the assassin to "acquire" some object or otherwise do something other than eliminate his target.  Assassins who fail in their contract seldom live to try again.  Those who find that the contract is too difficult or are otherwise unable to complete the task may return to the guild and petition the leaders for reprieve.  It is up to the guild masters to decide if another assassin must be sent, the same assassin is sent back out with backup, the contract holder contacted about supplying "inaccurate" information, or the assassin suffer sanctions for abandoning a contract.  Assassins who intentionally betray a contract, or otherwise make too many mistakes are marked for death by the guild.  For these individuals, there is nowhere to hide.  

Uncanny Dodge: Starting at 2nd level, the assassin gains the extraordinary ability to react to danger before his senses would normally allow him to even be aware of it. At 2nd level and above, he retains his Dexterity bonus to AC (if any) regardless of being caught flat-footed or struck by an invisible attacker. (He still loses his Dexterity bonus to AC if immobilized.)

At 5th level, the assassin can no longer be flanked, since he can react to opponents on opposite sides of him as easily as he can react to a single attacker. This defense denies rogues the ability to use flank attacks to sneak attack the assassin. The exception to this defense is that a rogue at least four levels higher than the assassin can flank him (and thus sneak attack him).

At 10th level, the assassin gains an intuitive sense that alerts him to danger from traps, giving him a +1 bonus to Reflex saves made to avoid traps.  This bonus increases by +1 for every five additional levels in this class.

If the assassin has another class that grants the uncanny dodge ability, add together all the class levels of the classes that grant the ability and determine the character’s uncanny dodge ability on that basis.

Assassins choose their spells from the following list:

1st level—change self, detect poison, ghost sound, obscuring mist, spider climb.
2nd level—alter self, darkness, pass without trace, undetectable alignment.
3rd level—deeper darkness, invisibility, misdirection, nondetection
4th level—dimension door, freedom of movement, improved invisibility, poison.

The Assassin prestige class first appeared in the Dungeon Master's Guide.
That class has no place in my campaign, this one overrides.