Quote from RainbowBriteI'll comment properly later tonight (I'm at work, can't really do it now), but for now I'll comment on Sin. It is overcosted. Drop it to XBB and make it 'Whenever Sin has five or more counters, it gains shroud'. You pretty much have to play it from your hand because of the X cost so 5BB for a 6/6 trample, shroud at the cost of 5 life isn't so bad and its versitility of being a 3/3 trample for 2BB at the cost of 2 life isn't so bad either. Apart from the cost, it's quite nice though.
Also, you should try and standadize the tokens. It seems that there is going to be lots of different ones which isn't good. Try and make at most 2 of each type (preferably 1). I'd personally go for all nighmare tokens being black 2/2's with fear and the fantasy's being blue/white 2/2's with flying. Remember that for bigger drawbacks you can put multiple tokens in.
Edit: I'm on lunch now, So I'll do what I can
Englufing Nightmares: Nice, but if you wantto try and standardize tokens, make it two 2/2 Nightmare tokens with fear.
Brigani: Fine, not overpowered not too weak either. Being lengendary stops it being strictly better than some cards.
The cycle of mana,T creatures: Limited filler, but the white one is lame. 9 mana for a wall? No thanks.
Cetil land: Nice, although I'm generally not a fan of too many legendary lands in a set. I like the fact that you can put nightmares into play under their control and then take them back.
Cruelty: First card with Dreamstate. Dreamstate seems that it could be done as enchatments.
At the beggining of your upkeep deal one damage to target creature.
2RB: Cruelty becomes a 3/3 Nightmare creature (this effect does not end at end of turn).
Ok, they are slightly different (countered in different ways, die to different removal,and the major thing: flavour isn't there.) But i think something needs to be done to make them a little more special (and be careful to to step on forecasts toes). Also, the reminder text needs to be sorted. I haven't got the time at the mo, but I'll give it a think.
Other than that, I like it. The flavor for dreamstate is very nice.
Fantasize: Mulldrifter? Very similar and possibly worse due to multicolour and token.
All I have time for now, I'll be back.
Quick edit (work is boring..): 2 cards for you, may need some altering,
Creature - Fantasy?
Protection from nightmares
Creature - Nightmare?
Protection from fantasies
You may want the Realist to be black and maybe change the creature types. Was just trying to go for flavor and have some ok commons. Hope you like them!