• It is a long time campaign and we should be arriving to ascension levels.
• Characters are currently Level 5 (3250xp in total).
• In the group from what I know there is a Sister of Battle (adepta sororitas) of the Dialogus kind (the diplomacy ones, not sure of the name).
• A melee psyker with a force blade.
• A techpriest and a regular one. (I do not know much about those two)

Remember that your stats are 2d10+20. I thought you'd rolled a little low at first glance.

If you want to go the guile and subterfuge route, I would advise against being an Untouchable. Untouchables get some crippling penalties to Fellowship (-10 reduction in Fellowship on top of a -10 to social situations), and from a fluff standpoint you'll be the creepy guy nobody can stand to be around. Calixian Pattern Killings is much safer and generally more useful.

As for weaponry, you might want something powerful but easily concealed, like a .54 Tranter or similar. Keep a longarm like a shotgun or autogun as a backup weapon hidden in an easy-to-reach place just in case you need it. Also, take a shock maul, perhaps upgrading to a power maul when you can afford it. Also, flak armor is your friend. It's cheap and offers decent protection until you can afford something better.

Schola training is good for Arbitrator. You'll be able to use Las and SP Basic and Pistol weapons, in addition to Primitive melee weapons, without spending XP. The less XP you spend on weapon training, the better.

If you want to play an untouchable you can consider a Sister of Silence type of character. I don't have the link to the PDF but it should be on dark reign if that website is still around.

Gitmasha didn't mention it, but he has personal experience with Moritat, they are tough to play unless you don't mind heavy restrictions on gear for a bonus which is amazing at the start but slowly loses its charm. For RPwise, they can be great, but know they are not power class, but there are ways to make up for their shortcomings. He was also right on nulls, don't forget they don't play well with psykers.

Schola is a great BG for martial characters, though not for social. Giving you a weaker version of a WP buff, free weapon training, free basic lores, but a social penalty which affects PCs in most campaigns.

As for weapons, the Tranter .54 has its good points, but a simple autopistol can be good. One of the best is the Puritan-14 which is a decent autopistol with a 1 shot shotgun built in, it fits the arbites theme (and is fluffed as such), will share ammo with your shotgun and you can have some fun ammo in it, like Blazer Shells (4 throne 15m Primitive flamers), slugs (1d10+5 Pen 2), plain scatter shells and much more. For longarms, an overpowered and cheap but amazing weapon is the Ironclaw, the best is the Vanaheim though. Autoguns are neat but don't fit the arbites style as much, if you want a powerful and flexible one try the cadence spectre, a 3x 20 round mag, a 4 shot underslung shotgun barrel and is fun (though overpriced),otherwise get an Armageddon for a higher caliber gun (loaded with manstoppers is solid) We can give more and better suggestions if you list your preferences and style. Also, guard flak is a good decent armor, but mesh may be better if your group prefers stealth.

Thank you Gitsmasha, Alox, and Cymbel, for your answers.

Darkreign doesn’t seem to be working maybe I’ll find the info on the forums there.

After reading your answers I think I’ll go arbitrator. The untouchable and moritat were choices based on an idea to make an assassin specialized in dealaing with the nasty Psyker-Sorcerors plague, all the same if you ask me… ; ). 

I’ll see with my GM if I can have an imperial world as origin as the fellowship malus is indeed quite annoying.

I’ll post later this day a draft build and ask my GM to know how much money I can have sothat the equipment can be done more precisely.

Thanks for the weapon advice. Stupid question though, what is the order of usefullness for criticals (X,E,I, …) ? 

Sang de Poulpe said:

First of all, please note that my advice is nowhere professional, it simply builds on play experience. And I'm not quite sure that we always play perfectly according to the rules, since the critical tables only tend to come up when PCs receive critical damage, while NPCs just get torn apart with when they reach 0 Wounds.

I don't think there is any usefull difference between the criticals for the various weapon types. Sure, it's great to have some sort of spectacular feedback of what your awesome roll just did to some poor hapless heretic, but in the end, the enemy is very much dead. Critical hits become interesting when they are happening to you, but in those cases, the weapon type is probably not your choice. 

Of course, this does not mean that you should ignore the weapon types that are at your disposal. My Tech-Priest uses las-weapons (E) simply because that's the way I like to play him, even though I am aware that other weapon types would probably deal a great deal more damage. (Unless I can get my filthy mechadendrites on a portable Lascannon or Multilaser… ahem…) The best is probably the Tearing quality of the bolt weapons, but I haven't really tried comparing other stats. :)

Sang de Poulpe said:

For more info on sisters of silence: http://warhammer40k.wikia.com/wiki/Sisters_of_Silence

The PDF: http://manuel.rifault.free.fr/Public/JDR/Dark%20Heresy/Sisters%20of%20Silence.pdf

It is an old file, so some of the new rules for untouchable may be preferred, YMMV. It could be a good idea to take the battle sisters from the Matyr book and modify them to silent sisters with inspiration from this PDF, i.e. throw out the faith stuff and put in the untouchable stuff.

My 2 Thrones

Be wary of Energy Weapons.  This is based purely on play experience: Randomly detonating ammunition and flaming headless corpses are just as dangerous to players as they are to NPC's.  I've never seen it kill an ally, but I've had allies go into critical damage from explosions and a couple who caught on fire.

Depending on your GM, funds, and whatnot you may want to look at the Arbites Suppression Shotgun found in Book of Judgement.  It always felt a bit munchkiny to me, but that's an attestation to the stats.  The Vanaheim is an excellent alternative due to the full-auto firing.  Never underestimate the benefit of Supressive Fire.

As for armor, Cymbel is right about Mesh.  It's a great middle-tier armor.  If you can afford it, and you won't focus on stealth, look to the Enforcer Light Carapace as a good middle-tier armor.

I'm not sure Sisters of Silence even exist after the Heresy anymore. Even if they do, they're probably restricted to guarding the Black Ships. Having one with an Inquisitorial acolyte cell is a waste of a good (and incredibly rare) operative.

A problem with the "special" Arbites shotgun is how hard it is going to be to find ammo for it, this doesn't use the normal "12 gauge" (or whatever the most common size is for 40k shotgun shells is), while a Vanaheim can take the standard ammo.

As for criticals, a VERY rough order from ingame experience is eXplosive > Rending > Energy/Impact

As IoE said, light carapace is great, but it sticks out. For budged medium armors, it goes

Light Carapace for Protection, Mesh for Stealth, Guard Flak for Budget (and it is 5 armor all around for blasts)

For Arbites, they have 3 background packages in IH and a rank 0 there too, there are a LOT of packages and special Arbites ranks in the new Arbites book and one in RH (for vigilante Arbites, as well as a black ops Arbites exectioner rank) and don't forget there are others they can take, but these are Arbites specific and have some great fluff and crunch to them

Cymbel said:

Good Point.  If one has regular access to Arbites stockpiles/armories the Arbites Shotgun is a good choice.  Otherwise the Vanaheim is probably the highest mark.

@Sang,

Bolt Pistol as a solid backup weapon.  Only use it when you need that extra kick.

Bolt Pistols can beggar even a wealthy acolyte when firing.

If you have a recoil glove, there are nice handcannons, including the Tranter .54 if you have SB4

The Orthlak Mk4 is a 12 Shot 1d10+4 S/-/6 Autopistol, put some manstoppers or expanders in that and you have a darn good backup sidearm

The Puritan-14 is a brutal and sloppy weapon, but has a shotgun mode which is a nasty surprise for foes, (especially with a slug, blazer or some of the other fun rounds)

The Armsman 10 is a solid 13 round semi auto, not amazing, but a decent backup if you don't want something fancy

there are several more, but this is a decent selection without knowing details or budget

I'm just curious, since I've never tried it before, but does Double Team, which only works in melee, function together with a cyber-mastiff?

If it has the talent, yes.

There is an example of one having it in the Inquisitor’s Handbook and the rules to add it to any cyber creature. In this case it is just an implant costing 500 Thrones. So you could upgrade any cyber creature to have it.

Not a big fan of the Impostor Background, especially with Arbites, I reccomend to look at the others, especially the IH ones (though the RH, Justice before Law is an interesting one if your methods are more vigilante)

On gear, 1/2 price is weak, especially for higher ranking players, why would you have the chump gear that a new arbites would get, you are getting a free cyber mastiff worth about 3k+ (for IH values), the BoJ ones are even more expensive. I can understand the GM not wanting super cheese, but there are limits….

Starting Gear:

Shotgun and 12 shells Sell, keep shells, alternate idea, turn it into a hackshotgun if this is a double barrel, see if GM is willing for you to buy a proper shotgun instead (at least pumpaction) if you sell at full value, otherwise 30 thrones… (Side Note, the Hack Shotgun is pretty darn good, especially if the GM lets you be able to fire it one round at a time as well as double mode, even without it, given quickdraw and maybe rapid reload it is a nice emergency gun and looks cheap too

Club-if you want a cheap hard hitter before shock, BQ mono upgrade is 85 thrones, 1d10+1+SB Pen2 +10 to hit

Brass Knuckles-Keep, BQ or GQ if you like fighting

Knife-Keep, Ditto

Mesh Vest - Could be good to keep, (150/2) = 75 thrones if sold, which is a ripoff

Stimm + Injector, this stuff can save your life, not often though, but it can be amazing

Arbites ID-useful for dealing with enforcers

Chrono-keep

Lhos/Amasec-Keep, how else can you fulfill some tropes?

If only 400 (and if you are doing an infiltrator) DO NOT get the Arbites special shotgun (either the HUEG round one or the awesome Persecutor), remember NPCs by a good GM are not dumb or blind, there are many shotgun options. The cheapest (and cheeziest) is the Ironclaw for 80 thrones, add a puritan for 100, stick with the mesh vest, get cheap armor to supplement, get whatever melee upgrades and weapon upgrades you can afford and some good gear

The 40k armory is pretty good for having ALL the things from ALL the books, for a real nice collection with some great fixes and price changes, check out the Macharian Handbook, has two books so far, a armor and weapons book and one with GEAR, see if your GM may want to use that, it really helps having almost all the items in one book (minus the heretical/xenos stuff overall, that will probably be another book), it is fan made, but VERY professional, though in the process of being updated for new books. Slugs and some  other  cool stuff (Bolts, Flechettes, Gas Rounds and Stun Shells for shotguns) comes from RT Hostile Acquisitions. It also has rules for making primitive armor lose the primitive status, assassin's longlas, shotlasguns, an arbites model awesome combat shotgun, making almost any armor into "powered" armor and those are just some of the GEAR options, it is a pretty fun book, like BoJ in my opinion, not something for everyone like IH/ItS, but if you have the cash, worth grabbing.