When answering this question, bear in mind I only have access to the DW book so I don't know what wargear in the other books is like.

I'm building a Storm Warden, and I like the idea of a space marine running around with a claymore. However, it doesn't seem like a good idea. Right at character creation sure, it'll hit hard, but it seems like dual-wielding will very quickly become better against single targets and groups of foes. This gets especially apparent when the Kill Team has the renown to afford lightning claws or power/chainfists and the dual-wielder is destroying everything, and I'm stuck with my starting kit. Even going sword n' board seems like a better idea.

I am aware that the relic blade is seriously awesome, but that is a long, long ways to wait. In the mean time a dual chainsword wielding is throwing out 3d10(pick the highest)+4, twice, versus a claymore's lonely 2d10+2. And the dual-wielder has much better armor penetration.

Are there traits that make two-handed weapons (which, as far as I can tell, is just the Storm Warden claymore and the DW relic blade) worth it? Am I missing something?

Concussive.

The end.

I know you said you don't have it, but Rites of Battle introduces a power claymore which basically bridges the gap between Sacris and relic blade. Wanna say it's Renowned.

Power Claymore:

Power Claymore* Melee 2d10+6E Pen6 Power Field, Unwieldy, Devastating (1) 18kg 15 Distinguished

Add 10 (or more) for strength bonus = 2d10+16 - respectable damage, full marks for style, and devastating (1) adds to the image of cutting a swathe through mooks.

Sounds like it would suit your character nicely.

That's a big help, thanks. The power claymore looks usable, and not terribly expensive.

And then perks. Are there any traits or abilities for two-handers? I've looked but can't find anything especially for that. And the same thing with sword n' boards- nothing beyond immediate benefit of having the extra save.

I am a firm believer that if you have talents that provide extra attacks using a two-handed weapon doesn't preclude one from making use of those attacks.

It is however all in how you choose to use them. If one can use an arm mounted weapon (they are hands free), multiple arm mounted weapons, and/or a combination of one-handed and arm mounted weapons using a two-handed weapon and an arm mounted weapon within the ruleset is a legitimate tactic.

With swift attack or lightning attack the character can make 2/3 attacks with one weapon and an additional attack with another weapon. That would be 2/3 blade swings with the power claymore and a pommel strike (an improvised weapon) or 2/3 blade swings and a shot from an arm mounted bolt pistol.

As I see it this is perfectly legal and is why no special rules for two-handed weapons exist. Sword and board style combat is the same as duel wielding since shields count as melee weapons (they just typically do less damage). There is no reason a space marine couldn't have a powershield if they wanted (in my opinion). There are shields that have the shocking and recharge attributes in Rogue Trader and Dark Heresy.

I recently acquired a lot of mono'd adamantium combat shields for our security team in my RT campaign to be used in combination with their chainswords to supplement their AP and provide a decent off-hand attack. So, I view them as a viable option at least.

Power shield = Storm Shield. Just give it a power field (including the +4 to damage and penetration that the power field property grants).

I think it is pretty easy to come up with cheap xp advances that make this a strong combo. Lightning Attack of course, three attacks at full WS. Add a good dodge and you're halfway home.

Some obvious talents are blademaster, counter attack, wall of steel and combat master, sure strike and precise blow. I'm not very familair with the Deathwatch talents.

Something I did consider was a talent similar to dual strike (perhaps a little less powerful) that had the same cost in prerequisites. I put the idea out there here:

http://www.fantasyflightgames.com/edge_foros_discusion.asp?efid=70&efcid=3&efidt=541880&efpag=0#542740

QUOTE:

I think that Dual Strike is probably a good option, but that means two weapon wielder too so it is a bit of an investment when he mostly wields a blade in one hand and a pistol/smg in the other.

I have been toying with ideas for a new talent. Similar in effect to Dual Strike but not requiring two weapons, it has similar or slightly higher xp prerequisties, follows the same rules and limitations, and has a similar or slightly lesser effect. Call it Vital Blow or what have you. Purpose is one well placed strike to deal with high toughness bonus. I do realise that this is a bit of a pandoras box with respect to balance, but Dual Strike with twin power axes is going to chop anything in half and I'm looking for something to keep me in the game without being totally defeined by all this high end gear.

Perhaps:
VITAL BLOW: Cost: 250xp. Prerequistites; Sure Strike, Blademaster, Armed with Melee(Primitive) blade and edged weapons. Effect: Using the same mechanics to attack as Dual Strike. Any Damage that gets through after toughness bonus is doubled. Limitation: Only one attack per round. This cannot be combined with dual strike or two weapon wielder in any way.

Or:
VITAL BLOW: Cost: 250xp. Prerequistites; Sure Strike, Blademaster, Armed with Melee(Primitive) blade or edged weapon. Effect: Using the same mechanics to attack as Dual Strike. Remove armour and then add an additional strength bonus multiplier to damage before removing toughness bonus. (e.g. if you have S40 and Unnatural Sx2 already, you add an additional +4). Limitation: Only one attack per round. This cannot be combined with dual strike or two weapon wielder in any way.

UNQUOTE.

The second option might wowrk for Deathwatch. Note xp prices and descriptions are Dark Hewresy focussed.

There's also the fact that every space marine has Unarmed Master, so a couple hits with the big sword and a severe pummeling with a fist/elbow/shoulder/knee/foot are perfectly viable.

I have to admit I am looking for a 2handed Chain Sword for Blood Angel Assault Marine.

Have I missed it somewhere?

I think I recall a chain glaive in RoB, or dig up stats for the Eviscerator from somewheres.

As to Talents for two handers, they do exist, i.e. Hammerblow & Furious Assault; since they're based off the All-Out-Attack Action, they're of limited use to dual wielders or sword n' board wielders, but are a good fit if you're looking for maximum hitting power with little regard to defence. My Storm Warden TM is having lots of fun with his Hammerblow/Claymore + Servo-Arm combo.

well it looks like the things your missing are;

1) your tables not using erratted weapons.

2) 2h weapons give you an extra stack of unnatural strenght to damage / parry

i found 1. the chain greatsword in Black Crusade

Now my GM needs to let me have one!!!