Actually I would change the converted mana cost of this card to 5. It's to expensive for my taste. ;)

Also here a few wording suggestions:

"When Karoon of the Cursed Crystal would die, transform him instead.

{2}{B}: Put a 1/1 black Nightmare creature token with shadow onto the battlefield (*Reminder text*).

{2}{R}: Karoon deals 2 damage to target creature. A creature dealt damage this way can't be regenerated this turn."

You can change the regeneration restriction if you want, so that it only applies, when Karoon would kill the creature with his ability. But I don't exactly know the wording for this case.

For reference:
Jaya Ballard, Task Mage: http://magiccards.info/pd2/en/10.html

Other side:

It's very unusual that a black/red creature has vigilance, this is a typical ability for white, sometimes green creatures.

Besides that, you should mention the name of the card in the regeneration ability:

"{1}{1}{B}: Regenerate Discord, the Wolf Demon."

Also if you want, that the transformed card has a color, you must use the color indicator. Reference: Howlpack Alpha http://magiccards.info/isd/en/193b.html
The transformed card has no mana cast and therefore no color if not mentioned otherweise.

Wow ^o.o^ Thanks a lot.

So glad to get tips from a professional ^.^ I am sure that I will get better in time. I barely ever played magic. Only have 20 Boosters worldwake xD Funny enough I drew Jace, Mind Sculptor.

Still i don't read nearly enough cards to be consistent with my texts ... yet

Also thanks for the tip with Vigilance.

I wanted to express that he just doesn't tire ... but on the other Hand, I might just go with trample after all, give him 5/4 instead of vigilance

As with your other tip I will have to wait until the evening, before I can make changes.

I am not really a professional. ;) I am just a active player for some years now and was sometimes interested in custom cards. There are some forums, where you can show your own cards for discussion and I learned from this stuff.

5/4 Trample with a regenerate ability looks decent, not too strong, not too weak.

Done then :) And yes, ... literally meaning you would have to be paid to be a professional. But it's obvious that you are far more experienced in that topic, than I am :) As for other discussion forums. If my aim was to create a whole 'edition' (which might happen) that has some consistency I would go there, but so far this is just to make a few people happy :) and playing custom with my friends.

I would assume that going into such a forum would be a rather rigid form of quality control, when it comes to balance. Maybe when I feel a little more accomplished.