Templar, to me, is probably a Secret-interaction (like Necromancer) whereas Mage is probably a caste like you say.
On the blood thing, it's a matter of changing their blood into a syndrome-carrier that does a transformation.
There's several steps to this bit:
1. Learn how to change tissues.
2. Learn how to create syndromes
3. Work out transformation syndromes.
So, if we look at the Giant cave spider raw, we see:
[USE_MATERIAL_TEMPLATE:POISON:CREATURE_EXTRACT_TEMPLATE]
[STATE_NAME:ALL_SOLID:frozen giant cave spider venom]
[STATE_ADJ:ALL_SOLID:frozen giant cave spider venom]
[STATE_NAME:LIQUID:giant cave spider venom]
[STATE_ADJ:LIQUID:giant cave spider venom]
[STATE_NAME:GAS:boiling giant cave spider venom]
[STATE_ADJ:GAS:boiling giant cave spider venom]
[PREFIX:NONE]
[ENTERS_BLOOD]
[SYNDROME]
[SYN_NAME:giant cave spider bite]
[SYN_AFFECTED_CLASS:GENERAL_POISON]
[SYN_IMMUNE_CREATURE:SPIDER_CAVE_GIANT:ALL]
[SYN_INJECTED]
[CE_PARALYSIS:SEV:100:PROB:100:RESISTABLE:SIZE_DILUTES:START:5:PEAK:10:END:20]
If we change this a bit:
[USE_MATERIAL_TEMPLATE:BLOOD:CREATURE_EXTRACT_TEMPLATE]
[STATE_NAME:ALL_SOLID:frozen tainted blood]
[STATE_ADJ:ALL_SOLID:frozen tainted blood]
[STATE_NAME:LIQUID:tainted blood]
[STATE_ADJ:LIQUID:tainted blood]
[STATE_NAME:GAS:boiling tainted blood]
[STATE_ADJ:GAS:boiling tainted blood]
[PREFIX:NONE]
[ENTERS_BLOOD]
[SYNDROME]
[SYN_NAME:tainted blood]
[SYN_AFFECTED_CLASS:GENERAL_POISON]
[SYN_INJECTED][SYN_CONTACT]
Now we've got our starting set up for "tainted blood".
[USE_MATERIAL_TEMPLATE:BLOOD:CREATURE_EXTRACT_TEMPLATE]
[STATE_NAME:ALL_SOLID:frozen tainted blood]
[STATE_ADJ:ALL_SOLID:frozen tainted blood]
[STATE_NAME:LIQUID:tainted blood]
[STATE_ADJ:LIQUID:tainted blood]
[STATE_NAME:GAS:boiling tainted blood]
[STATE_ADJ:GAS:boiling tainted blood]
[PREFIX:NONE]
[ENTERS_BLOOD]
[SYNDROME]
[SYN_NAME:tainted blood]
[SYN_AFFECTED_CLASS:GENERAL_POISON]
[SYN_INJECTED][SYN_CONTACT]
[CE_BODY_TRANSFORMATION:PROB:100:START:10:CE:CREATURE:TAINTED:GHOUL]
This should (if I've done it right) transform anything that comes into contact with the blood into a ghoul. BUT, in Dragon Age Taint affects different races different so...
[USE_MATERIAL_TEMPLATE:BLOOD:CREATURE_EXTRACT_TEMPLATE]
[STATE_NAME:ALL_SOLID:frozen tainted blood]
[STATE_ADJ:ALL_SOLID:frozen tainted blood]
[STATE_NAME:LIQUID:tainted blood]
[STATE_ADJ:LIQUID:tainted blood]
[STATE_NAME:GAS:boiling tainted blood]
[STATE_ADJ:GAS:boiling tainted blood]
[PREFIX:NONE]
[ENTERS_BLOOD]
[SYNDROME]
[SYN_NAME:tainted blood]
[SYN_AFFECTED_CREATURE:HUMAN:ALL]
[SYN_INJECTED][SYN_CONTACT]
[CE_BODY_TRANSFORMATION:PROB:100:START:10:CE:CREATURE:TAINTED:GHOUL_HUMAN]
[SYNDROME]
[SYN_NAME:tainted blood]
[SYN_AFFECTED_CREATURE:ELF:ALL]
[SYN_INJECTED][SYN_CONTACT]
[CE_BODY_TRANSFORMATION:PROB:100:START:10:CE:CREATURE:TAINTED:GHOUL_ELF]
[SYNDROME]
[SYN_NAME:tainted blood]
[SYN_AFFECTED_CREATURE:DWARF:ALL]
[SYN_INJECTED][SYN_CONTACT]
[CE_BODY_TRANSFORMATION:PROB:100:START:10:CE:CREATURE:TAINTED:GHOUL_DWARF]
We have to make ghouls affected by this syndrome, too, otherwise they'll rapidly switch in and out of ghoul state (which is crappy).
But now we come across another problem! What about the rare chance that a female becomes a broodmother? Well, we need more transformations but we also need to make them immune to the ghoul transformation, so what we do is add in syn classes and make the broodmothers immune to them (we also have to make the ghouls able to transform into broodmothers):
[USE_MATERIAL_TEMPLATE:BLOOD:CREATURE_EXTRACT_TEMPLATE]
[STATE_NAME:ALL_SOLID:frozen tainted blood]
[STATE_ADJ:ALL_SOLID:frozen tainted blood]
[STATE_NAME:LIQUID:tainted blood]
[STATE_ADJ:LIQUID:tainted blood]
[STATE_NAME:GAS:boiling tainted blood]
[STATE_ADJ:GAS:boiling tainted blood]
[PREFIX:NONE]
[ENTERS_BLOOD]
[SYNDROME]
[SYN_NAME:tainted blood]
[SYN_AFFECTED_CREATURE:HUMAN:ALL]
[SYN_AFFECTED_CREATURE:TAINTED:GHOUL_HUMAN]
[SYN_INJECTED][SYN_CONTACT]
[SYN_CLASS:GHOUL]
[SYN_IMMUNE_CREATURE:BROODMOTHER_HURLOCK]
[CE_BODY_TRANSFORMATION:PROB:100:START:10:CE:CREATURE:TAINTED:GHOUL_HUMAN]
[SYNDROME]
[SYN_NAME:tainted blood]
[SYN_AFFECTED_CREATURE:ELF:ALL]
[SYN_AFFECTED_CREATURE:TAINTED:GHOUL_ELF]
[SYN_INJECTED][SYN_CONTACT]
[SYN_CLASS:GHOUL]
[SYN_IMMUNE_CREATURE:BROODMOTHER_SHRIEKER]
[CE_BODY_TRANSFORMATION:PROB:100:START:10:CE:CREATURE:TAINTED:GHOUL_ELF]
[SYNDROME]
[SYN_NAME:tainted blood]
[SYN_AFFECTED_CREATURE:DWARF:ALL]
[SYN_AFFECTED_CREATURE:TAINTED:GHOUL_DWARF]
[SYN_INJECTED][SYN_CONTACT]
[SYN_CLASS:GHOUL]
[SYN_IMMUNE_CREATURE:BROODMOTHER_GENLOCK]
[CE_BODY_TRANSFORMATION:PROB:100:START:10:CE:CREATURE:TAINTED:GHOUL_DWARF]
[SYNDROME]
[SYN_NAME:tainted blood]
[SYN_AFFECTED_CREATURE:HUMAN:FEMALE]
[SYN_AFFECTED_CREATURE:TAINTED:GHOUL_DWARF]
[SYN_INJECTED][SYN_CONTACT]
[SYN_CLASS:BROODMOTHER]
[CE_BODY_TRANSFORMATION:PROB:100:START:10:CE:CREATURE:TAINTED:BROODMOTHER_HURLOCK]
[SYNDROME]
[SYN_NAME:tainted blood]
[SYN_AFFECTED_CREATURE:ELF:FEMALE]
[SYN_AFFECTED_CREATURE:TAINTED:GHOUL_DWARF]
[SYN_INJECTED][SYN_CONTACT]
[SYN_CLASS:BROODMOTHER]
[CE_BODY_TRANSFORMATION:PROB:100:START:10:CE:CREATURE:TAINTED:BROODMOTHER_SHRIEK]
[SYNDROME]
[SYN_NAME:tainted blood]
[SYN_AFFECTED_CREATURE:DWARF:FEMALE]
[SYN_AFFECTED_CREATURE:TAINTED:GHOUL_DWARF]
[SYN_INJECTED][SYN_CONTACT]
[SYN_CLASS:BROODMOTHER]
[CE_BODY_TRANSFORMATION:PROB:100:START:10:CE:CREATURE:TAINTED:BROODMOTHER_GENLOCK]
As you can see, with this we end up with a series of syndromes that allow for a transformation. You should probably also gender differentiate them for the sake of it, but otherwise this will work.
Another option is to make the affected creatures similar to the Animated dead in default DF, but I think that's less worthwhile.
Come join the Bay12Games Discord (
https://discord.gg/dfeSEVX ) if you're interested in learning more about modding. We've got ZM5 in there and he's forgotten more about modding in this game than I've managed to learn.