The nnightmare clan is a great clan its one of the more balanced clans. Many clans depend on there power to heavily or there damage or the clan bonus nightmare depends on all three so takeing one away from them like the clan bonus doesn't hurt them to bad so there good for fusion decks particularly Gheist and piranas.
Okay the nightmare clan's Sob is there bread this makes them very effective against clans that relies on the clan bonus like Rescue or montana so if you fighting someone who uses such clans nightmare not a bad choice.
Now that you have a grip on the basics of this clan lets talk about abilities nightmare's primary ability its life manipulation such as +2 life per damage or -4 opp life min 4 poison so a couple cards that are staple's in just about any nightmare deck are Glorg is a stopper and a damagers great for everything Kenny he can make a gap of 15 points enough said, Ghumbo awesome for later rounds, nistarok awesome Dr and powerful power and five damage is pretty good and Ohshitsune ability makes him almost unstoppable seven damage possible nine i'd say yes an essential these cards are absolutely essiential your gonna need some them if your serious about playing a good but not essiential filler card is Pan his +3 life and 3 damage make him a great three star his flexibility is what makes him so marketable in this case good for bluffs and can be a game changer another filler i'd suggest getting for two reasons is cheapness and awesome ability Sheitane. Next up for defense is Nerfeniti. Everything else is just up to what you play style is and how big you budget is so good luck with these few tips will help out.