Pisces

Two Angler Fish. Since they're NightGhasts, their fins and tails are covered with a flow of nightmares (darkness -3). Their lures have globes attached at the end.

Big

Water/Dark or Nightmare

Red, Blue, Indigo, Purple, Grey, and Black for both fish.

I didn't decide if the attacks affect the whole party, several, or only one member.

When only globe is emerged:
1- Flail: The angler fish flails the party with its globe. Physical attack, neutral.
2- Wave: The angler fish thrusts its globe in the water producing a wave that attacks the party. Physical attack, water.
3- Flash: The angler fish flashes its globe with a bright light in succession. Status inflicting. (Possibly blind or such.)
4- Water Slice: The angler fish creates a water blade that attack the party. Magical attack, water.
5- Dark Energy: The angler fish creates a wave of dark power that attacks the party. Magical attack, dark or nightmare.

When provoked:
1- Bite: The angler fish uses its jaws to attack the party. Physical attack, neutral/dark. (This means that the dark is only effective if the member(s) of the party hurt are weak to it. If a member or members is/are resilient to dark, it is not taken into affect and changes to neutral.)
2- Tidal Wave: The angler fish jumps and slams back down into the lake creating a huge wave that attacks the party. Physical attack, water.
3- Flash: The angler fish flashes its globe with a bright light in succession. Status inflicting. (Possibly blind or weakens damage dealt, or such)
4- Water Lock: The angler fish produces a orb of water and traps a party member or members within. The member(s) contained receive damage each turn and may only use magical attacks, which deal less damage. Magical attack, water.
5- Water Blade: The angler fish creates a water blade that attack the party. Magical attack, water. More damage than Water Slice.
6- Trap Lunge: The angler fish submerges and globes appear at every waterfall. After a set number of turns, the angler fish lunges from a waterfall and attacks the party. Physical attack, neutral/dark.

I prefer 5 attacks and a signature move, but here's another:

5- Dark Sonar: The angler fish creates a wave of dark power that attacks the party. Magical attack, dark or nightmare. May inflict a status alignment. More damage than Dark Energy.

You could include the "Flash" attack mentioned in the main attacks but I wasn't sure where to put it.

1- Globe Shift: The angler fish changes the color of it's globe from red to blue or vise-versa. Later on, it changes the colors depending on an element and tints it red or blue.
2- Globe Extinguish: The angler fish forces its globe to stop glowing, but in return it's damage dealt is increased.

Jaw. (Teeth, fangs, whatever you decide.)

This battle starts around a deep lake with several waterfalls falling into it (Maybe 4, more or less, read on to see why). You could use some sort of symbolism that the waterfalls feeding the lake are dreams or part of the fear. Pisces is dormant deep in the lake feeding on the dreams and turning them into fears, or just feeding on fears.

At first, only two globes emerging from the water glowing differently. A red glow means that the respective globe can be damaged by Physical attacks. Magical attacks will damage it, but they'll be weak. A blue glow means that the respective globe can be damaged by Magical attacks. Physical attacks will damage it, but they'll be weak. The two globes may alternate between red and blue during the battle. At this stage, with only the globes emerged from the lake, they fish will only use simple Physical and Magical attacks. The difference is one fish is more resilient to Physical attacks, regardless of globe color, while the other is more resilient to Magical attacks.

The Damage when attacking the globes won't affect the actual NightGhast, when enough damage is done to a globe, the respective angler fish provoked emerges. The globes of the fish will still be glowing, but when a fish is provoked to emerge the globe of the fish will sometimes stop glowing. When this happens, the angler fish who's globe stopped glowing will normally receive both Magical and Physical damage without penalties such as when a globe glows.

When provoked, they attack with various Magical and Physical attacks outlined in the above form. When a fish is provoked, it will sometimes use the attack "Trap Lure". This is the bosses signature attack. The provoked fish will submerge and several globes will emerge from around all the waterfalls. These globes will be glowing either red or blue. The trick here is that one of the globes is the "right" one, a certain amount of damage must be dealt to the "right" globe. After a set number of turns after the attack is begun and the player has failed to damage the right globe enough, (I have no idea if the enemies are animated so I'll just throw this in) the angler fish will lunge out from a waterfall and severely damage the party. (Or...you could choose a certain number of members or just one.) The angler fish will use this attack more when their in critical state, when their HP lowers below a certain amount. Resilience to the opposite attack type is increased, and Physical damage of the fish is increased.

After one fish is killed, the remaining fish will submerge into the lake and fully heal. (Yes, I'm sorry.) After it emerges, it will repeat the above cycle, but this time the globe will glow different colors to indicate different elements, but the catch is that the globe will also glow with a tint of red or blue. For example, if the globe is now glowing brown with a tint of blue, it's damaged by earth elemental Magical attacks. When provoked, the fish stays in critical state and the new globe rule from above is applied.