10 sourcebooks to choose from, which do you recommend and why?
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Chaositech (Malhavoc Press)
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(minor spoilers for Ptolus)
Chaositech is a dark version of magitech. It plays somewhat important role in Ptolus setting.
If you're familiar with WFRP, this is about CHAOS-tainted [1] mechanical constructs and body replacements (mix 3 parts of Giger with 1 part of WFRP and 1 part of s-f implements).
The chief fault of the system, IMHO, is that it is so dangerously unpredictable and unrewarding, that one would have to be stupid, instead of simply insane to use it.
Ingredients are costly, difficult to obtain, prone to breaking or running out of fuel (yes, that's right, I said "fuel"). Implementing them is seriously risky for the health of everyone involved (both the patient and the surgeon/constructor) and even if you get it, you just get tainted.
In a world of reliable magic crafting, it does not make sense.
The good thing is that it is easy to improve. Just make it an easy road to mid-level power (i.e. nothing beyond 4th level of spells in terms of power, require great/big/huge engines to accomplish anything beyond that), keep the fuel angle, make the permanent taint a remote possibility (and temporary one a real and present danger) and you're set.
Given all of this, it's pretty recommendable.
Good synergy with Ptolus and various supplements for Ptolus.
Regards,
Ruemere
[1] That's not CHAOS as in Chaotic, but CHAOS as in "utterly alien, thoroughly evil and totally bent on absolute destruction of reality".