Unnatural Characteristics changed rather significantly from deathwatch to Black Crusade and I felt most of it was a very positive change. the one thing that bothers me though is that individuals with Unnatural Characteristics have no better chance of succeeding at there tasks than those without them, Yes if they do pass a test they get extra degrees of success but only if they already passed the test. This leads to superhumanly strong and tough creatures who are no more likely to shake off the effects of poison or stunning than those who lack there enhanced abilities. It also prevents such superhuman individuals from ever barely passing a test its either failure or a minimum of multiple degrees of success. (space marines either fail a toughness test or get 3+ degrees of success)

In Earlier Versions of these rules Unnatural Characteristics lowered the difficulty of tests but due to the changes in the way Unnatural's work this is no longer easily done. So my hack for this system is actually very simple remove the last line of the trait. Unnatural Characteristics always adds 1/2 there value in bonus degrees of success to all rolls, Degrees of failure cancel out degrees of success on a one for one basis. Zero degrees of success is a failure.

Hence 1-2 Unnatural Characteristics have the same success odds as none 3-4 is roughly equal to 10% higher stat, 5-6 is around 20% exc. This allows marines and augmented, marked, and mutated humans to fail by a small margin and still barely succeed. It makes a naked average space marine more likely to win a grapple than a normal human with above average strength.

Now this could have some unseen problems with some stats though they should be minor the only one I would address right now is those odd psyker powers like warptime that grant unnatural characteristics. Most people seem to think warp time is a bit overpowered as is and with this house rule would be even stronger so I am just going to have warptime and most powers like it grant 1/2 psy rating rather than full, perhaps 1/2 round down. then again I think I would rather see warptime as granting rerolls but thats something to look at another day.

Now on to Multiple Arms I always hesitate when I come across something that grants extra attacks in deathwatch it was strong but not brutally so generally just one more attack bringing most creatures with it up to 4 if they had lighting attack. 4 possible hits of which with the right talents 3 might be avoidable with luck. In black Crusade though it grants 1/2 its value in extra attacks all of which can be lighting attacks and only two of those can possible be defended against in most cases. Not only is that extra deadly but also slows things down when something like a genestealer is making 4 full auto style attacks each causing multiple hits its alot of rolling. I am happy to have multiple armed opponents be very dangerous but perhaps a bit less so and a bit faster.

My house rule for Multiple Arms is a quick rewrite

Multiple Arms (X)

This creature has more than one pair of arms. The exact number of arms is indicated in parenthesis (X), So a Genestealer with Multiple Arms (4) has four arms total. It gains a +10 bonus on Athletics tests involving climbing, and swimming. In addition for each pair of arms it has--the Multiple Arms value divided by 2--- it gains bonus degrees of success on all Swift Attack and Lighting attack actions. A Creature with multiple arms and two weapon wielder gains this bonus on both attacks. A creature that lacks the talents Swift Attack and/or Lighting attack may still take the swift attack action due to there multiple arms. In the unusual event of a creature wielding multiple different weapons in there multiple arms they must choose one to make each attack with the extra weapons are assumed to be used to create openings and assist the primary weapon.

not sure i needed to include all that stuff about different weapons in the multiple arms trait but i wanted it to be clear.

so how awful are these house rules and how and why do they break the game?

I like them. I was just thinking to myself that you could house rule Unnatural Stats so that they gave a bonus equal to [1/2 Stat * 10] to any test with that stat. But it is significantly different than what you are suggesting, and potentially much more powerful.

I like the multiple arms work you have there. I was thinking of something similar for interaction between Two Weapon Wielder and Lightning and Swift Attack. Essentially it would work like Twin Linked, anyone making a Lightning Attack or a Swift Attack with TWW would get a +10 bonus to their attack and an extra hit for every 4 DoS, separate from the extra hits from Swift of Lightning Attack. Like your Multiple Arms rules, attacks would only land with one weapon. The extra hits from TWW could be made with any weapon used in the attack, the choice is up to the player. I'm not entirely sure if a rule like that is necessary though, I do know that it would reduce the number of rolls needed for an attack to one though, rather than rolling for two separate Attacks.

I would personally keep two weapon fighting at two attacks to line up with the two defenses (standard plus, Step aside) that a character can get. if you do change it to one make sure to change how two weapons and ranged attack change to keep them consistent. the one issue I could see would be a individual with just two weapon wielder who lacks swift or lighting attack would make two attack or possibly one with extra successes for 4 degrees (though that is highly unlikely to happen with any regularity and makes defending against it very easy) and then gaining swift attack would suddenly change it to one attack. just a bit odd.

I'll admit its imperfect and somewhat wonky when you consider Standard Attacks and firing two ranged weapons in some situations. It does make Unwieldy weapons a little bit more useful though.

For unnatural characteristics, why not just give a +5 bonus/Unnatural characteristic to related checks? That would make you more likely to succeed and give you 1 degree of success for every 2 points of Unnatural characteristic. It also makes Unnatural (1) Useful in that it at least gives a +5.

Space Marine gets +20 to all STR checks, so they can "barely succeed". I'm not sure how that would affect game mechanics. It might make some things with unnatural Agility impossible to hit, or Marines Impossible to grapple…