Yes, what I meant is that creatures did not die from a disabled heart, and died from a disabled brain but only because of breathing issues
Ah yes, organs in dwarf fortress work really weirdly. If I recall correctly the heart isn't a vital organ at all, so a creature with it disabled will live on just fine, and the brain's main use is to keep the lungs from continuing to work, hence the suffocation when disabled (I think a creature without breathing parts might be able to survive just fine with a function-disabled brain).
May summoning a larger creature which will attack to give the syndrom be a solution ? edit: what about a contaminant ?
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I tried several severity values : it seems to have no effect at all above 134050 (I have no idea why this particular number), is this a known maximum or something ?
Syndromes and concentrations are a bit of a mess that I don't fully understand. Depending on how they're administered, the concentration can vary dramatically as far as I can tell. Injection syndromes I
think take into account the size of the creature administering (possibly versus the size creature being injected with it), while contaminants possibly varying in strength based on the size of the contaminant(?) which is hard to really gauge or control.
Seeing as you're looking for an effect that instantly kills a creature, why not upgrade that death ray into a disintegration ray? I noticed that the wiki has
an example of a creature that instantly disappears into a puff of smoke - you could have your ray effect transform victims into that creature instead. That way, you don't have to worry about syndrome concentrations, since the CE_BODY_TRANSFORMATION effect doesn't take them into account. The only negative is that I don't know if it'd work on creatures that are currently transformed, though that's fewer caveats than going with a death method.