@ Spiryt: Thanks for the constructive criticism. I'll try to answer to the best of my abilities

Quote Originally Posted by Spiryt View Post
Drain blood... You mean ALL blood? you know it's dimunitive, how much blood can it drain?
It never intended the creature to drain all the blood of a creature. At most it can drink about ten litres (from two average humans), but I do not believe it possible for a leech to drain a creature completely of blood, so I'm thinking it would be around six litres. This is quite a lot for a diminutive creature, I agree, which is why I noted in the heartworm's advancement that a 4 HD heartworm is of tiny, and not diminutive, size. It will, of course, be very bloated at this point. I'll consider clarifying this, and please tell me if you still feel it is unrealistic.

Quote Originally Posted by Spiryt View Post
But yeah I know it's magic and all. But anyway one constitution pere round is very fast, he IS diminutive after all.
Well, a dire weasel drains 1d4 constitution a round, and its not built for blood draining the way a cross-breed between a heart and a leech is. It is medium-sized, though, so I guess the leech is still a tad quick. I'll have to thank it mostly to the fact that I'd like to keep the timescale of the blood drain within the timescale of a D&D combat.

Quote Originally Posted by Spiryt View Post
It looks like surprising attack is quit deadly for not very hard humanoid(around 4-5th level), dc 16 is not so easy.
I never was very good at balance, and I made the DC that high to ensure that heartworms actually had a real hope of making it to becoming a horror. If the DC was lower, the paralysis not working would be quite common, and if the paralysis doesn't work, a heartworm is in fact quite boned. It is slow and squishy, and if its paralysis failed more than 1/4 of the time on a commoner, its likelihood to survive would sink drastically. All I can say to my defence is that balance was never what I focused on. I'll consider changing it if I get more cries of outrage

Quote Originally Posted by Spiryt View Post
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?
Well, it is at least the size of a frog (a normal leech would be 'fine'), and I suspect having that kind of weight hanging on your body would alert most people, and its jaw would probably pack quite a bite. Also, as you say, blood drain of this magnitude would be quite noticeable.

Quote Originally Posted by Spiryt View Post
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)
I forgot to include a paragraph on how to identify/remove it *slaps forehead*. Thanks for the heads up. I'll add that in, but not today. Getting late.

Quote Originally Posted by Spiryt View Post
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...
Hmm, energy drain was a rule I was a bit unclear on. I thought level drain allowed no save, and the save was just to see if the level would remain drained at the end of the day? Am I mistaken here? I don't want to include a mechanic that depends on natural 1s to work. I'll need to read up on energy drain...

Quote Originally Posted by Spiryt View Post
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.
Ah, well the constitution damage is more of a timer for how long time it takes for a character being consumed to die, not the time it takes for a horror to eat all of his/her organs. I see that including the "keeping alive as long as possible" bit might lead one to such conclusions. I suspect the consuming would take quite a bit longer, but that it would take about one minute (Con 10) to die from it. Also, it is true that this ability doesn't take possibilities such as armour into consideration. I am to blame for always envisioning some poor, filthy peasant when picturing the victim. I'll consider expanding this rule soon.