I heard my name?
This is a good card. Fits darkness;s parasitic/symbiotic effect nicely. Harpy would fit the effect well indeed.
Anyways balance:
Skill:
Assuming the statistics Crow took on cards in hand is accurate then the average cards in hand is 3.25
Assuming this were in a deck then Nightmare would be as well. Over the course of a game the opponent's hand would fill up with more unplayable creatures and thus add to this creature's attack.
Estimated average = 4 (subject to playtesting in a nightmare deck)
Hence estimated value for the skill is 4
Upgrade:
4HP -> 5HP is way too small of an upgrade.
4HP -> 6HP is acceptable.
0|4 [N|4] -> 1|4 [N+1|4] would work
0|4 for 4
-> 0|4 for 3
would work as well.
This is a well balanced card.