Assassins: Assassins first appeared in the OD&D boxed set as an NPC specialist, and in Supplement II: Blackmoor as a playable class. They appear here as a subclass of the Thief. It's slightly easier to become an Assassin in AD&D: you still need a Strength and Dexterity of 12, but the Intelligence requirement has dropped from 12 to 11. The racial restriction has also been loosened: in OD&D only humans can be Assassins, but here the class is available to every race except for halflings.
The alignment restrictions have changed as well. OD&D Assassins were all neutral, but now they are required to be evil. Personally I agree with the latter, but it raises some interesting ideas about how the Assassins' Guild was previously run. If killing people for profit is intrinsically evil, how could the older breed of Assassins be neutral? I have two thoughts here: either the old-school Assassins had an ultimate goal (maybe a benevolent one) that went beyond killing for profit; or the way the original alignment system worked didn't take into account the morality of killing for profit. (I'll be covering the alignment chapter in a few entries, and I might get into the relationship between the three alignment systems presented so far.)
As a Thief sub-class, Assassins have all the abilities of that class at two levels below their Assassin level. The only Thief ability they can use without this penalty is the Backstab. They can also use shields, as well as every weapon on the equipment list, so although they have the same combat abilities as Thieves they're slightly more versatile.
Assassins can use poison, either in their victim's food or on their weapons. There's a chance that a bared weapon with poison on it will be spotted by those nearby. In OD&D that chance was 50% per round, but in AD&D it's been changed to 10% cumulative chance per round. The NPCs in AD&D are also less aggressive in their reactions: they might attack or call the city watch, whereas in OD&D they would all attack ferociously. Why none if them run like buggery is anyone's guess.
The ASSASSINATION TABLE (Gary's emphasis) is mentioned, in that an Assassin who surprises an enemy can use it to try and kill the victim instantly. That sounds great, but the table isn't included! Presumably it's in the Dungeon Masters Guide, which is fine for me, but I feel sorry for the folks who had to wait two years for the bloody thing to come out. (Admittedly, the text says that it gives the Assassin a roughly 50% chance of an instant kill, so at least there's something to go on. That sounds awfully high to me, I must say.)
The ability for Assassins to learn new alignment languages is kept from OD&D, but the rules have been changed to accommodate the different alignment system. The Assassin needs a 15 Intelligence to learn a new one, and can learn an extra one for every point of Intelligence above that (to a maximum of four). Thieves Cant and Druidic are also included on this list. (This goes against my ideas about alignment languages: that they're the primal languages from the dawn of the universe, and that there's something about them that makes them anathema to those of different alignments. I guess whatever it is that makes them so, the Assassins' Guild has figured out how to get around it.)
Their disguise ability has also been retained - Assassins can change their appearance, even appearing as members of the opposite sex or a different race. Relevant NPCs have a chance of spotting the disguise every day, and this number is modified by Intelligence and Wisdom. It works basically the same way that it did in OD&D, except that it seems to me that AD&D Assassins are harder to spot.
A table is given for the cost of hiring an Assassin, with the price determined by both the level of the Assassin and the level of the victim. (In OD&D, only the Assassin's level was taken into account.) If you really want to, you can drop 10,000gp to get a 15th-level Assassin to murder a 0-level NPC. It costs more to have important people like nobles murdered.
Probably the most important factor of playing this class is the Assassins' Guild. The book says that there is a Guild in "most towns and cities", which is shockingly far-reaching. The demand for assassination must be pretty high. An Assassin PC doesn;t have to be a member of the local guild, but if a non-member does any assassinating in that guild's territory, they will be marked for death. Each guild is headed by a Guildmaster Assassin of 14th level. It's not made explicit, but it seems as though these Guildmasters report to the Grandfather of Assassins, and to me it sounds as though there's only one such Grandfather in the whole world.
For any Assassin to reach 14th level, he has to either murder a Guildmaster, or challenge him to a duel. The same goes for becoming 15th level, with the Grandfather of Assassins. This is similar to the way that Druids and Monks advance at high levels, but trickery and deceit is actively encouraged for Assassin duels.
The level titles for Assassins have been significantly changed and rearranged from those in OD&D. There are still the old menacing ones like Murderer, Killer and Executioner, but there are now a few that are lovably silly, like Rutterkin and Waghalter. The Grandfather of Assassins is new in AD&D. OD&D Assassins advanced in level a bit quicker.