This card is mass removal that varies based on untapped and tapped creatures, but you can protect your own creatures by using them to convoke this card
Scions & Spellcraft is a decent deck out of the box, but between the four Commander decks, it's the one that will need the most upgrades to match the power of other Commander decks
It won't increase or decrease if the number of Forests you control later changes
One good way to work with this is to be playing necro valley and some decks having GY triggers or being able to operate on low life isn't a good excuse for 6k burn or +5 in card advantage
Given that Duskmourn introduced a bunch of enchantment nightmares, including a couple that didnt quite make the list, you could look to enchantress synergies like Sythis, Harvest's Hand and Inquisitive Glimmer for a more concentrated way to use your nightmares
Raigeki is the card that monster cards wake up in a cold sweat dreaming about