Attention mate, you cannot use Nightmare packs in Splinterlands, they're from Soulkeep, which is a different game.

I would honestly go for Chaos Leagion packs if you don't have a lot of money to put in. Otherwise Rebellion, because Chaos Legion will rotate out of Modern format in a few months.

You could also save money at the moment and then go for Conclave Arcana packs once they will be released. You will be able to use them for the longest period in Modern, because they will be brand new.