Ghostmare decks are of course the most powerful combo to benefit.
Lets look at the ideal situation: Your opponent has zero cards in his hand. You have an upgraded Ghost of the Past on the board. You have a hand of 8 cards, 1 nightmare, and 1 Mind Rot.
1 Nightmare = 16 damage
You play Mind Rot, so you discard 6 cards = your opponent discards 6 cards = 78 damage
Total damage = 94
Wow that's pretty powerful. Except how likely is that to be the case?
For one thing, if your opponent knows you are playing Ghostmare (likely with the Ghost on the board), your opponent will probably be playing more carefully to keep extra cards in his hand.
For another, how likely are you to have 8 cards in your hand? I suppose you could add Hourglasses to increase your draw rate.
Still, it's more likely that you will only get 4 to5 Ghost into the opponent's hand with a Nightmare, and you'll probably be discarding 5 cards at the high end, and even less if you are trying to play the Ghost on the same turn as the Nightmare and Mind Rot.
That does the following:
1 Nightmare for 5 Ghosts = 10 damage
Mind Rot discarding 5 cards => discard 5 random cards from your opponent's hand, so lets say 3 Ghosts + 2 other cards = 39 damage
Total of 49 damage
Counters include Sanctuary, Shard of Sacrifice, Nightmare (on the Ghost, so that your opponent will be discarding Ghosts), Reverse Time and other CC on the Ghosts, Fractal/Hourglass/Sundial/Precognition (to keep your deck full).
Plus, if you have any Ghosts of the Past in your hand that you discard you will also take damage for those cards (and that damage would be applied before the damage to your opponent).
Now that is still really powerful, but you do have to get a 3 card combo in specific situations.
Is that too powerful? As I said that is one of my worries. The easiest change would be to increase the cost of the card, the other possibility would be to place this card in a different element so that you have to play a Trio deck to use it with Ghostmare. If this is too powerful in a Duo deck then I think I'd prefer the second solution and consider moving it to Entropy or something. That would weaken the combo with Ghostmare, while not significantly hurting other possible combos.