Acolyte and Metamorph leaders can add a bonesword to their weapons choices. But should they? And if they do, when should they use it?
The bonesword only appears in three places in the current codex: acolyte leaders can swap out their cult knife to get one; metamorph leaders can take one in addition to their other weapons; and the Primus gets one in its default load-out. In all three cases, this means that anyone carrying a bonesword could also choose to use a rending claw instead (unless you give a metamorph leader twin talons and a bonesword, but we’ll ignore that for today’s purposes).
Carrying both weapons means that we need to make choices, and when we need to make choices, we can math out the advantageous option in any particular set of circumstances. So let’s do that.
RENDING CLAW vs BONESWORD
The claw and the sword are very similar. Neither confers any WS or strength bonuses; both do 1 damage. The only difference is in AP: the sword has a flat -2AP, whereas the claw is usually -1AP, but rises to -4AP on a 6 to wound.
Here’s the chance of landing a damaging hit with a single claw attack:
And similarly, with a single bonesword attack.
On both tables, I’ve highlighted in green wherever that weapon has an advantage, and in orange when it’s worse off. White squares show where both weapons are equally likely to cause damage. Against lightly-armoured targets (6+ save or worse), there’s no difference between claw and sword – the AP on both is sufficient to remove the save completely. But for saves of 5+ or better, we can see that both options have areas of comparative strength.
The rending claw matches or beats the bonesword against any targets of T8 or better, and is always more effective once those targets hit a 4+ save or better. It’s also the way to go against targets in the T5-7 range, as long as they have a 3+ save or better. So if you’re going after tanks or very heavy infantry, the rending claw is the good choice.
The bonesword matches or beats the claw against any targets of T4 or worse, and is more effective if those targets have a 5+ save or better (apart from T4 and 3+/2+, where it’s back to parity). It’s also better against T5-7 with an exactly 5+ save, which is quite a specific profile. If you’re facing low toughness targets, or lots of 5+ saves, the bonesword might be worthwhile. That said, with the exception of T2 targets, it’s only gaining you between 2-5% better performance, which may not be worth the 5 points the bonesword sets you back.
SO, WHAT GIVES?
So why the split decision? Given that the weapon profiles don’t change, and that AP is really the only distinguishing feature, why do both the claw and sword have distinct areas of advantage? It’s because the rending effect (6s to wound pushing the AP to -4) gets relatively more effective the tougher the target becomes.
Against a T3 target, a successful claw hit will wound 4 in 6 times. Of those four times, an even distribution of results would give us three regular wounds and one rending wound – that’s three wounds at -1AP, and one at -4AP, for a mean average AP of -1.75, or less than the bonesword’s default -2AP. Whilst the claw will generate occasional spike damage via rending (assuming a T3 target has sufficient armour for that to even make a difference), it’s too erratic compared to the consistent -2AP of the bonesword.
As the target toughness increases, the chance of scoring regular wounds decreases; but the chances of rending remain fixed at 1 in 6. This means that the relative chance of getting that -4AP improves as the overall chance of wounding decreases. Across an even distribution of results: at T4, one third of successful wounds will rend (for an average -2AP), putting the claw on par with the sword; at T5-7, half of all successful wounds will rend (-2.5AP); and at T8+, all successful wounds will be rending, because we’re only wounding on 6s anyway – at this point, a rending claw is effectively a -4AP weapon.
[For clarity – it’s not that it gets easier to wound with a claw as the target toughness increases. It’s about the relative chances of a successful wound also being a 6 and triggering the rending effect. Against a T5 target, where only 5s and 6s wound, you’d expect two thirds of all wound rolls to fail; but of those that succeed (the 5s and 6s), you’d expect half of those to be 6s.]
I think this shifting AP explains why the bonesword is slightly better against T4 targets with a 5+ or 4+ save, and T5-7 targets with a 5+ save. A 5+ save is the bonesword’s comparative sweet-spot, where it gains the full benefit of -2AP to completely remove the target’s save, without any of that AP being wasted. We’ve already seen that, against T4, the claw’s average AP matches the sword; but we don’t get the full benefit of that -4AP until we hit a 3+ save.
To look at it another way – imagine hitting something like an AdMech ranger (4+ save, but with a 6+ invulnerable). Anything above -2AP is wasted, soaked up by the invulnerable. So the bonesword would be ideal here. On the other hand, the claw becomes less effective, as it either hits at -1AP, or rends but is limited to an effective -2AP. At best, it does as well as the sword, but never better.
CONCLUSIONS
It’s important to remember that the unit options we’re looking at here aren’t exclusive. Anyone who can carry a bonesword also has a rending claw. Every time our acolyte or metamorph leader fights, they can opt for the weapon best suited to whatever they’re hitting in the moment.
It's worth noting at this point that we are talking about very marginal gains, here. When we say that one weapon is better than the other, the margin is usually around 2-3%. There's not a dramatic advantage for either option. Not that this should stop us messing around with numbers, but it's good to keep things in perspective.
The bonesword is a bit of a ‘bully’ weapon – it outperforms the claw against units you’re already good against, in that it’s better against targets whose toughness is lower than your strength, and just about edges it when strength and toughness are equal. The rending claw, meanwhile, is a ‘hero’ weapon, for those moments when you want to take on something tougher than yourself. It won’t make you great against tanks or super-tough infantry, but it’ll probably do better than the bonesword.
A further factor to consider is the strength of the acolyte or metamorph. As strength increases, the targets against which the bonesword is better increase in number (because when a weapon is good at punching down, increased strength gives you more people to punch down at). We could use the Helix creed for an easy S5, with the potential to buff that to S6 with Might from Beyond or the Icon of the Cult Ascendant, or even reach S7 with all three in play. By which point, you’ll want to be using the bonesword against anything with T7 or less (which is most things). But you're using a lot of resources to get there.
And finally, the acolyte leader is giving up its cult knife (and its extra attack) to gain the bonesword. Not a huge deal, but in those places where claw and sword balance each other, you might as well have another free hit with a knife. So there’s that to consider too.
I usually run my Cult as Helix, and also tend to make use of Might whenever I can. So my current preference is to stick boneswords on the leaders of my big acolyte blocks (not least because it makes them easier to find on the battlefield). But two boneswords could be another rock saw somewhere else, and you might find that to be a better use of those points.
Given all of the above, and assuming that we want to go for the most effective and point-efficient choices, I don’t think the bonesword is worth it. Yes, it’s marginally better against light infantry, but it means taking an attack away from an acolyte leader (because they sacrifice the cult knife for the sword), and I suspect the points are better spent elsewhere. Then again, hyper-efficiency isn’t everything, and boneswords do look cool. And that’s important too.