Oof, I'm with Shade.
Ok, I think this is going to be messy, but I have a proposed plan.
1) Write a gaseous or incorporeal monster (at this point, I'm ceasing to care which) with an infusion special attack and phylactery type object (though why a golem would need a phylactery is beyond me, what a weird monster!). Am I missing any special ability here?
2) After infusion, rather than a modified alternate form/change shape, apply a template to the character "infused" to create a duplicate. This should have a "Con countdown" SQ. I vote that the "blood drain" or whatever it should be called should be a touch attack with a "do not notice" ability. We can flavor it to be incorporeality, but make it a normal touch attack mechanically.
Sound ok?
The disembodied base form should definitely have hp, mental abilities, etc. As for hazards, somehow they seem more iffy to me just because they're nonstandardized. And they are usually immobile, which would be tricky for this one.
13HD, Medium seems fair enough. And those abilities work for me, at least to try it out. I'm not sure about much with this critter!

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.