Just something to add on Robin's post, being the die-hard assassin fan that I am..
Poison has the effects of a rapid acid damager. What I mean is that it deals damage extremely quickly, and it is also able to kill creatures like trolls or foes shrouded in protective magics that only elemental damages can affect.
Granted, I played an evil Fighter/Assassin, multiclassed with Shadowkeeper (so sue me
) but they most assuredly do not suck. Thieves are not intended to be fighters, they are intended to hide in the shadows and backstab. You DO get a THACO bonus when you backstab from the shadows, so hitting should not be a big problem if you have a decent strength. I feel that using long-range weapons defeats the purpose of the assassin's strength (the super high backstab mod) and while tossing poisoned bullets and darts to your heart's content might defeat some enemies, you are not making full use of the assassin.
On the choice of race, I agree with Robin, Half-Orc might work, admittedly I've never tried but it might, but what I thought prudent was to make him an elf to get the DEX bonus to compensate for the lack of Thievery skills. I never had a problem with 15 skillpoints a level, because of the thievery bonuses of my DEX. They start high enough to get you through most of early game, and certain skills like setting traps you can wait till the numbers are up there in Hide in Shadows and Move Silently. Those should be your priorities, then Finding Traps, then Setting Traps. Having Hide in Shadows up in the earlier levels might detract from your effectiveness as an all-rounder thief, but you'll be able to wear elven chains later on if you're a fighter/assassin like me, and thats where having those skills up will come in useful.
Robin did not mention that trap killing does not give you exp for the creature you killed, so you'll still need to get your hands dirty. Without that exp, you won't get your assassin to the levels you need to handle stuff by yourself. I find the assassin very versatile in that he can handle anything, and anyone, even mages without the help of support because of his poison ability, which works best against mages and which you get multiple usages of.
So just an example in actual play..
Mindflayers, which many people find hard to kill all fell one by one to me by my backstabs, assisted by my Cloak of Non-Detection, which you find early in the game, and my Ring of Invisibility and Boots of Speed. Instead of the Boots of Stealth, just pump your skillpoints high in the Stealth department and reap the benefits of the Boots of Speed, which allow you to run and hide behind corners, hide in the shadows again, then backstab another flayer, the run and hide, and repeat. I did not get hit once except in combat with the Umber Hulks.
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Why?