It's a bit troubling that the Rogue gets more sneak attack dice! No sane Assassin would take level 20 in this class -- he'd take a level of Rogue to try & catch up.
Thats part of the reason I think Assassin is a Prc in the first case. I like assassin better without spells (rangers also don't get spells, in my brain). For a base class assassin you can just add spelllike abilities once per day to simulate spellcasting or ignore spells altogether. If I were going to do an Assassin class it would look something like this:
Assassin
Hit Die d6.
Requirements
To qualify to become an assassin, a character must fulfill all the following criteria:
-Alignment Any evil.
-The character must kill someone for no other reason than to join the assassins.
[If this is a starting character, there must be a murder in the backstory to begin as a level 1 assassin]
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), Diplomacy (Cha), Disable Device (Int), Disguise (Cha), Escape Artist (Dex), Forgery (Int), Gather Information (Cha), Hide (Dex), Intimidate (Cha), Jump (Str), Listen (Wis), Move Silently (Dex), Open Lock (Dex), Search (Int), Sense Motive (Wis), Sleight of Hand (Dex), Spot (Wis), Swim (Str), Tumble (Dex), and Use Rope (Dex).
Skill Points at Each Level: 4 + Int modifier.
BAB: As Rogue
Saves: Bad Fort, Good Reflex, Bad Will
1 Sneak Attack +1d6, Poison Use
2 Imrpoved Initiative, +1 Save Against Poison
3 Sneak Attack +2d6
4 Death Attack, Uncanny Dodge, +2 Save Against Poison
5 Sneak Attack +3d6
6 Invisibility 1/day, +3 Save Against Poison
7 Sneak Attack +4d6
8 Improved Uncanny Dodge, +4 Save Against Poison
9 Sneak Attack +5d6
10 Hide in Plain Sight, Invisibility 2/day, +5 Save Against Poison
11 Sneak Attack +6d6
12 Improved Death Attack, Poison Immunity
13 Sneak Attack +7d6
14 Improved Invisibility 3/day
15 Sneak Attack +8d6
16 Crippling Strike
17 Sneak Attack +9d6
18 Improved Invisibility 4/day
19 Sneak Attack +10d6
20 Ultimate Death Attack
Class Features [copied from SRD]
All of the following are Class Features of the assassin class.
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.
Poison Use
Assassins are trained in the use of poison and never risk accidentally poisoning themselves when applying poison to a blade.
Sneak Attack
This is exactly like the rogue ability of the same name. The extra damage dealt increases by +1d6 every other level (1st, 3rd, 5th, 7th, and 9th). If an assassin gets a sneak attack bonus from another source the bonuses on damage stack.
Improved Initiative
The assassin gains this feat for free at level 2.
Save Bonus against Poison
The assassin gains a natural saving throw bonus to all poisons gained at 2nd level that increases by +1 for every two additional levels the assassin gains. At level 12 an assassin becomes immune to all poisons.
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). 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 a Fortitude save (DC 10 + 1/2 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 2 levels 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.
Uncanny Dodge (Ex)
Starting at 4th level, an assassin retains his Dexterity bonus to AC (if any) regardless of being caught flat-footed or struck by an invisible attacker. (He still loses any Dexterity bonus to AC if immobilized.)
If a character gains uncanny dodge from a second class the character automatically gains improved uncanny dodge (see below).
Invisibility (Su)
At 6th level, an assassin can cast invisibility as a spelllike ability once per day. Every 4 levels thereafter he can cast it an additional time per day and at level 14 all uses are upgraded to Improved Invisibility.
Improved Uncanny Dodge (Ex)
At 8th level, an 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).
If a character gains uncanny dodge (see above) from a second class the character automatically gains improved uncanny dodge, and the levels from those classes stack to determine the minimum rogue level required to flank the character.
Hide in Plain Sight (Su)
At 10th level, an assassin can use the Hide skill even while being observed. As long as he is within 10 feet of some sort of shadow, an assassin can hide himself from view in the open without having anything to actually hide behind. He cannot, however, hide in his own shadow.
Improved Death Attack
Once the assassin has completed his study, he can make a death attack on the subject without further study as long as he does so within 1 hour.
Crippling Strike (Ex): At 16th level, the assassin can sneak attack opponents with such precision that his blows weaken and hamper them. An opponent damaged by one of his sneak attacks also takes 2 points of Strength damage. Ability points lost to damage return on their own at the rate of 1 point per day for each damaged ability.
Ultimate Death Attack
At 20th level an assassin can make an ultimate death attack once per day. If the ultimate death attack succeeds it results in death or paralysis as usual. The DC of of this attacks is (DC 10 + the assassin’s class level + the assassin’s Int modifier). A victim that successfully saves against an ultimate death attack gets a +10 circumstance bonus against any future ultimate death attacks.
However even if the victim succeeds against an ultimate death attack, the assassin can make another death attack without the required 3 rounds of study. This next death attack must be made within one minute of the ultimate death attack and the assassin must be able to make a sneak attack to trigger the death attack. This attack is made at the regular DC.
). It's easy to fix, though (thanks for pointing it out, however).
