First you need a special trait for the unit design, make one up but they all need it. Then add this

              <ProductionRequirement>
        <Type>Resource</Type>
        <Attribute>TestRes</Attribute>
        <Value>1</Value>
        <Provides>This unit costs 1 Testres per member</Provides>
    </ProductionRequirement>

The above makes it require 1 Testres per member when you train, and it will continuously drain that resource as long as the unit lives (if damaged it still drains the max amount, ie 2/3 members alive = drain 3).

Then set up a new resource like below

   <ResourceType InternalName="TestRes">
        <DisplayName>Fire Shard</DisplayName>
        <Type>TestRes</Type>
        <Description>Increases the power of your Fire spells.</Description>
        <IconColor>0,0,0</IconColor>
        <Icon>Gfx//Icons//Icon_Fire.png</Icon>
        <HideInHiergamenon>1</HideInHiergamenon>
        <ShownInGlobalDisplay>1</ShownInGlobalDisplay>
        <Global>1</Global>
        <Stored>0</Stored>
        <Shared>0</Shared>
        <TradedByCaravans>0</TradedByCaravans>
        <RummagedPerTurn>0.01</RummagedPerTurn>
        <!-- AI Info -->
        <AIData AIPersonality="AI_General">
            <AITag>Shard</AITag>
        </AIData>
</ResourceType>

Then all you need is to add the production of the resource to somewhere. Here I added 3 production to each tower of dominion a player controls:

   <ImprovementType InternalName="TowerOfDominion">
       <GameModifier>
            <ModType>Resource</ModType>
            <Attribute>TestRes</Attribute>
            <Value>3</Value>
            <PerTurn>1</PerTurn>
            <Provides>+1 TestRes</Provides>
    </GameModifier>
    </ImprovementType>
 

This would allow you to build 1 unit with 3 members (per tower of dominion), but nothing else. As long as the unit is alive, you can't build another one.

Well, looks like you nailed it.  Just did some testing and it certainly appears to do exactly what I want.

I thought I had already tried that same setup... possibly the only thing I might have done differently before, is that I think I previously had the ProductionRequirement against the unit directly rather than the ability, can't quite remember.  I'm wondering what difference that makes.... actually thinking about it, the bound widow has a production requirement of like 20 mana so why doesn't that contionually train 20 mana?  Is it because the cost is against the unit directly rather than the ability?

Since henchmen are always just 1 member, you can instead just simply remove 1 influence per turn, like the opposite of merchant trait.

The "trick" to making the sort of resource-upkeep we did above actually lies in the resourcetype, in particular this <Stored>0</Stored> which makes the game treat the resource as continuous (like shards) instead of stockpiling (like metal). There was some code used in E:wom for food for that type of resource that allowed us to create the unit scenario we did above here.

OMG it works a treat, this is awesome.  I'm trying to upload a pic but this forum software is so rubbish...

The only danger I see is it possibly going negative, which would limit further production but not cause anything else to happen (players may expect them to disband?).

Hmm lemme try.....

Ok cool, got it after a bit of mucking about.  I was trying to use the https dropbox url before silly me.

Thanks!

I bet that string is the formatted description 

Ah, good work HF!  Just tested, well you *can* actually get it to work if you add an entry to elemental.str, eg in my case this:

[JB_CultSerpent_Influence_Description] Foobar.

And you get this:

However doing that would break my rule of editing anything in the install folder.  And I presume it would be a bit of a no no it terms of support, and probably get overwritten every patch so a bit crap all round really

Thanks for pointing me to it though.