This is kinda scary
And so coplex and damn interesting that ik can only say:
MitP Vote: YES

However i have some doubts:
Quote Originally Posted by Ceres View Post

Attach (Ex): A nameless heartworm that hits with its bite attack sticks itself onto the creature with its glue. An attached heartworm can be struck with a weapon or grappled itself. To remove an attached heartworm through grappling, the opponent must achieve a pin against the creature.

Blood drain (Ex): A nameless heartworm drains blood for one point of constitution damage each round it remains attached. When the heartworm has drained the blood of one medium-sized or larger creature, two small creature or four tiny or larger creatures, it gains a Hit Dice. The creatures drained must have a soul, or the heartworm will just vomit up the blood.
Drain blood... You mean ALL blood? you know it's dimunitive, how much blood can it drain?
But yeah I know it's magic and all. But anyway one constitution pere round is very fast, he IS dimunitive after all.
It looks like surprising attack is quit deadly for not very hard humanoid(around 4-5th level), dc 16 is not so easy.
And what when it bites but not paralyze? Is it easily noticable so can be quickly killed or is it worst tick? I supose that
losing blood in such rate is noticable but what with bite itself?

Determining that you have Weeping soul soul remains high The planes knowledge i suspect? There are probably some forms of acting to avoid death before its too late (heavy depression)
Energy drain (Su): Every round a creature with a soul is grappling with a nameless horror, they gain one negative level. The DC is 10 for the fortitude save to remove the negative level. The save DC is Charisma-based. If a creature reaches level 0 due to the energy drain, it dies and the horror gains a stolen feature (See below). The feature is stolen from the corpse, and it will look on the corpse as if it was born without it. The horror has stolen the creature’s soul, and it cannot be raised or resurrected unless the nameless spirit within the horror is destroyed.
DC 10 is preaty easy on the other hand, but when you are paralyzed 2-5 minutes, you will probably loose so many levels even due to natural 1s, that you won't avoid being hit/grappled even if you survive...

And Consume...

It's totally not your fault that D&D do not have any rules for consuming, but still consuming is too fast IMO and loosing "only" point of Con/round due to being eaten so fast is little too small thing. And armors, for example medium and heavier (but better lights too, armors are durable by definition after all), they will surely interrupt consumption, and since there are no reasons why not to even sleep in light armor, this will be kinda hard to consume somebody.
Definetly not many humanoids wear armors all time/at all of course.

OK this is all damn niggling, but when developing such complex rules, doubts will be