One thing i've noticed is that there is, as far as i can tell, no traditional weakness of undead to priests that you see in d&d. So my suggestion is to add to the decks of undead creatures some black drawback cards: "Unholy": (trait, duration 2) Healing this creature instead damages it for twice the value of the heal.
I like it, also it could apply to priests because they have unholy cards too. I think demonic revenge should do 3 unpreventable damage instead of 4 of it, because cards like ouch(2 damage for each attack) and traveling plague (5 damage then go to enemy deck) can have their damage prevented.
I like the idea, fits in very nicely with already established rpg tropes. Rather than causing double the heal amount as damage, I would rather make it so: "Unliving": (trait, duration 3) If you are being targeted by a healing effect, you receive an equal amount of unpreventable damage instead. I would name the trait "Unliving" (rather than Unholy) because the vulnerability to healing spells traditionally only applies to skeletons, zombies and other kind of undead creatures. A creature being tied to the "Unholy" sphere doesn't immediately imply a weakness to those kind of effects, although I have to admit D&D isn't extremely consistent with what is vulnerable to which effects anyways. Also, making it double the damage would make undead-themed adventures (woodhome, tvericus, etc) all too trivial. I.e. a 3 priests party would easily make short work of whatever happened to have the handicap card attached to them. Similarly, cards like Combustible and Vulnerable makes you take additional damage (rather than multiplying whatever damage was originally incoming) for the very same reason. Furthermore, if such an handicap was added to priests equipment, you would need to prevent potential damage to go sky-high because of MP balance consideration. I think simply making it unpreventable damage is more reasonable while still keeping the effect interesting by giving it a twist. Alternatively, you could make it discard a card every time you receive damage from a healing effect, although that would probably be too powerful still. Not sure if Blue Manchu will ever take the time implement the card and add it to all the undead creatures' decks, but this still is a nice idea nonetheless.
Could always be added in future expansions. Would love to see some big bad vampire undead challenge. There's already plenty of existing cards they can use out of a vamp priests traditional card compliment. Other cards like regeneration traits, a stun attack(gaze), and some strong bash attacks. Add to that a new armor that prevents melee damage plus this new weakness to healing and you would force players to shift their deck style to include a healbot or wizard build to deal with a very tough vampire enemy.
To make it an effective conceptual addition, it'd have to be added to each unholy/healing cleric card. EG, minor heal would become 'Heal target 2. If target is undead, instead deal 2 holy damage.' If you want it to be a randomized vulnerability, similar to combustible on trees, then we should perhaps go with a 'Curse of the Undying: Trait (3) When you take Unholy damage, negate that damage and heal the amount that would have been dealt. When you are healed, negate that healing and take damage equal to the amount that would have been healed.'
