Playing as an evil party

The game rewards you for being evil more so than being good. with that said i almost always have had a banite DG cleric in my party to reap the benefits of evil only items as well as the reward for a certain banite related quest. it is really strange that such a character(non human or banite) would oppose the legion seeing as they both serve bane. however there are several instances where this interaction comes into play into the form of interesting dialog at various instances in the game.

however evil, i don't go around slaughtering people as long as their death doesn't cause me a game over. i also don't accept the offer of a certain evil deity because it would not make sense no matter how evil the party.

 

Wealth, fame, power and maybe revenge could be possible motivations for an evil group. The party is referred as mercenaries or sellswords in numerous occasions, so no problem there - they're in for personal gains. Besides, being evil doesn't necessarily mean killing innocents and eating babies for breakfast. By the way, there's a dialogue in Targos where the group is asked why they helped the city.

If I remember right, the Legion serves Bane's son more than Bane itself and the opposing Banites are some sort of heretics.

I don't remember playing an all-evil party myself, but one of my favorite characters was a LE fighter(4)/dreadmaster(x). I loved his dialogue choice before the final battle, "kneel before the black hand of Bane" or something like that. Very nice touch

 

All Evil parties are well rewarded in IWD2. Any build will work in the Vanilla game, it's too easy. I usually prefer no Multi-class if possible when playing this game in vanilla, making it more challenging. I usually use the following party or something similar.

Human Cleric(Bane) - Tank, Healer, Summoner, Intimidater and Party Leader
Half-Orc Cleric(Tempus) or (Gruumsh) if you use mods. - Tank, Summoner and Healer
Half-Orc Barbarian - Tank Tiefling Rogue - Locks/Traps, Bakcstabber, Monster Gatherer and Crossbowmen.
Human Necromancer - The epitome of Evil - Summoner and Blaster
Drow Bard - Buffer, alternate Speaker
or instead of the Bard...
Deep Gnome Monk - Tank, Spell-absorber, Monster Gatherer

I have also used an all Deep Gnome Party, which is tons of fun. Little, evil DGs runnnig around, SLOWLY wearing the enemey down while absorbing or dismissing the damage they are taking. Fun, but I find it a slower game all in all.

For something different, I have also tried an all Half-Orc Evil Partial Barbarian party. It was weaker in Arcane Magic, but a Melee monster.

Fig(4)/Barb
Barb(1)/Cleric(Tempus)
Barb(1)/Cleric(Tempus)
Barb(1)/Sorcerer
Barb(1)/Wizard
Rogue(5)/Barb

As for coming up with a reason for the party to exist/be in Targos, that is simple. Any reason will work. For the Gold and Treasure is the first thing that pops into my head. I am sure Ulbrec wouldn't mind that 6 Evil Half-Orcs save his town. Beats a Goblin Axe in the head any day. Kuldahar is under siege when you get there, so the saying "The enemy of my enemy is my friend..." goes without saying. The inital reason could be the gold/loot, but as they get dragged in, it could be revenge on the twins.

 

There is a Drow later on in the game which tells you quite simply why an evil party would be willing to take down the legion. Many typically evil races and individuals will have a sense of purity of purpose, for the Drow this includes perfection of the race, developing a society of mongrels means the strengths are few and the weaknesses plenty. Tieflings might resent the demeaning of both fiendish and mortal parent races but overall they are individuals more than a society.

Elves, Humans, Dwarves, all would probably have a sense of foreboding when it comes to the prospect of being forced into a relationship of an external species just to make things interesting, given that dwarves are notoriously hard to cross breed they may become second class citizens in a world of mongrels, humans may feel taken advantage of as in most situations "half" means "human X hybrid" in D&D, Elves probably just think it's icky.

 

You could say that the reason why your evil little group was sent off to Icewind Dale in the first place was because of something that happened in Luskan (the PC and co. are Luskanites, iirc).

Anything is possible it comes to backstories involving Luskan.