I'm trying to figure out, how would I have to go about building a Rogue/Champion of Torm character, and have trouble with understanding the requirements for the Champion of Torm class (or at least I'm not sure if I get it right).

According to http://nwn.wikia.com/wiki/Champion_of_Torm page, I'll need the Base Attack Bonus of +7 to be able to pick the class. On the http://nwn.wikia.com/wiki/Rogue#Level_progression page it says that Rogue will have +7 BAB at level 10.

So, does it mean I won't be able to fulfill the prerequisites for Champion of Torm until rogue's level 10, or are there some other factors influencing how BAB is calculated?

BAB is determined exclusively by your character's class(es) and levels. Each character class follows one of three BAB progressions:
-Combat class: +1 BAB every level
-Semi-combat class: +3/4 BAB every level (technically, this means that you get +1 every 3 out of 4 levels)
-Non-combat class: +1/2 BAB every level (technically, you get +1 every second level)

So, to answer your question, you indeed need to be at level 10 to take CoT levels if you're a rogue, but, for example, if you were a fighter, you'd only need level 7.

You can find more info on BAB here: http://nwn.wikia.com/wiki/Base_attack_bonus

Hope that answers your question. :)

Yeah, it does all right, thanks. :)

Edit: I now wonder though, how does it work for multiclass characters?
Let's say I'd be level 10 Rogue and level 11 Champion of Torm, would the BAB of each class just add up? (So would I have +18/+8/+1*, or some other values of BAB?)

*CoT 11: +11/+6/+1, Rogue 10: +7/+2

The base attack bonus of each class is rounded down and added together, and iterative attacks are calculated based on the combined total. Your iteratives are capped at 20th level; they can continue to become higher numbers, but you cannot get more slashes.

As an example, a level 10 Rogue has 7 BAB, and a level 10 Champion of Torm has 10 BAB. This adds up to 17 BAB for the character. To get the iterative values, you continue to subtract 5 until you hit 0. So the next value down is 12, then 7, then 2, and then we can't go any lower so that's the last slashed value. This means you'd get +17/+12/+7/+2 as your full attack.

Once you hit 21st level, however, you're considered an "epic" character and the rules work a bit differently. Your base attack can continue to increase, and your iterative attack bonuses can increase, but the total number of iterative attacks (slashes) does not. So, let's say that at 20th level you're a Rogue 10 / Champion of Torm 10 and have the +17/+12/+7/+2 iterative spread I just talked about. Let's suppose you keep leveling up as a Rogue past this point. At level 25 you will be a Rogue 15/Champion of Torm 10, which will give you 11 BAB from Rogue and 10 from Champion of Torm for a total of 21. Because you only had 4 slashes at the 20th level you can't get a 5th, even though your base attack bonus is now high enough to qualify for one. This gives you the attack bonus +21/+16/+11/+6. Technically speaking we could subtract 5 once more to get another slash of +1, but because of the epic rule you're stuck with only 4.

Another example based on rounding. Suppose you have a bard 1 / rogue 1 / sorcerer 1. The bard has 3/4 progression (rounded down to 0), the rogue has 3/4 progression (rounded down to 0), and the sorcerer has 1/2 progression (rounded down to 0), so the character has 0 base attack bonus. Be mindful of this when multiclassing; rounding works against you.

To add to what Darvin said, at epic levels (21 and beyond) BAB progression is universal for all characters and increase by 1 every two character levels.

So, if you plan an epic-level character multiclassing build, it's much more rewarding to level as the high BAB class up to level 20 (if possible) and pack all the low BAB levels once your character is epic.

For example, let's consider a fighter/wizard build for level 40.

If you take your first 20 levels as a wizard and further 20 as a fighter:
-your BAB at level 20 will be +10
-at level 40 your BAB will be higher by 10, so it will be ultimately +20

However, if you reverse this order:
-your BAB at level 20 will be +20
-at level 40 your BAB will still be higher by 10, so it will be +30

As such, the order of class you level in is very important for epic-level builds.

Thanks for the tip. :)

Though I also recall reading, that once you hit character (not class) level 20 (or probably 21, to be precise), you get to pick epic feats for your class when levelling up.

So, in your scenario, Taro94, you wouldn't be able to pick fighter's epic feats, only the wizard's?
(unless advancing as a fighter past 20 some more, I suppose)

Not sure how would I handle it though, I suppose it'd depend on which feats I'd want, if making this detailed character plan.

BTW, how does it work with HP? I mean, other D&D games I recall (like Baldur's Gate II), past some level, only added 3 HP (regardless of CON) for warrior-type classes (fighters, rangers, paladins), and less for the rest (though multiclass levelling also worked differently, XP was divided evenly between classes, and each class levelled up once it reached the necessary XP amount, you weren't deciding which one to advance - so you weren't making this sort of decision, unless making a dual class human, who at some point permanently stops levelling as one class, and starts as another, in that case warrior-type class was always better to pick as the first, if included in the combo, for bigger HP pool).

Anyway, what I meant to ask before going off on a tangent, if I'd be making that Fighter/Wizard 20/20, and say, I would put levels into Wizard first, would I still be getting full 1d10 (+CON bonus) amount of HP every new level of a fighter??

Every class gets a random number of hit points determined by their hit dice. Fighter has d10 hit dice, so he gets 1-10 hit points, Rogue has d6 so he gets 1-6, and so forth. At the 1st level you are guaranteed the maximum possible result, and after that it's random. You also get a bonus (or penalty) to your hit points based on your constitution bonus. The total number of hit points gained at a level cannot be less than 1, no matter how big the penalty gets.

This never changes, and hit points continue to add up as you gain more levels. The Baldur's Gate series is based on AD&D rules (2nd edition) whereas Neverwinter Nights is based on 3rd edition rules. Yup, you'd still get 1d10+con at every fighter level regardless of when you take the fighter levels.

Constitution increases will retroactively increase your hit points. If this weren't the case, then magic items that grant bonus constitution would be useless. Intelligence is the only stat that doesn't give a retroactive bonus.

For building a character especially a multi-class character you might want to look at this excel spreadsheet. It will let you build and see the stats for almost any character you want to make: http://neverwintervault.org/project/nwn1/other/tool/characterbuildcalculator-cbc

That's the D&D rules, but it works differently in NWN. The game is a lot more forgiving.

Firstly, you get max hitpoints for the first THREE levels, not just the first level.

Secondly, the dice roll cannot ever be less than half the maximum. Low numbers are re-rolled. So, if you're a fighter with a D10 hitpoint die, you actually get between 5-10 hitpoints per level, not 1-10. A rogue gets 3-6 HP, not 1-6.

Finally, there's an option in an .ini file that lets you automatically get maximum hitpoints every level. This, obviously, makes the game much easier. (Also, it's possible to get max hitpoints even if this setting isn't enabled, simply by cancelling your level-up process if you get less than the maximum on the roll. Some online settings frown on this behaviour, though.)

Edit: Here is the full explanation.