There was a time back when Nightmare did not drain HP and Ghost of the Past didn't exist. It was considered horribly UP due to being unable to find a valid use in any deck.Ninja'd.
As it currently it, it is perfectly fine, IMHO - you don't want to target the opponent's field with this most of the time because you may accidentally give them free card advantage in the long run, a la Fractal.That leaves your field, which means you'd need to target high cost creatures in case your opponent has a Nova/Immolation handy.
By itself, it tends to be:
* a mediocre healing spell (at best on a low-card hand, comparable to a Holy Light, ~1
)
+ * a denied draw (Comparable to Precognition ~2
. Note that Nightmare doesn't cantrip - rather, it denies a draw, which is similar to Reverse Time, ~2
. Nightmare doesn't take a creature off the board either.)
, since discarding a card usually means nothing unless you're using Ghost of the Past. (A specific 1-card interaction that Nightmare needs to be fully viable.)
Overall, Nightmare is a weak denial spell that combos really well with a particular card, like Discord + Black Hole and Archangel + Rage Potion. I don't think it needs a change at the moment.