In this guide ill go over everything you will need to know if you want to succesfully kill The Nightmare and Night terror.

Main attacks:
The Nightmare has 4 attacks that will follow you throu the whole fight. (Grasping claw is a special attack and wont occur that often)
                                
Phase 1 specials:
Flowers: 
The Nightmare will divide the room into four equal quadrants with lines of flowers. One quadrant will be safe as indicated with white nightmare flowers, while the remaining quadrants are unsafe as indicated with red nightmare berries. Standing in the wrong quadrant will deal 5 dmg every tick.


Husks:
The Nightmare spawns two husks around random players in her arena. The targeted players will be frozen in place until the husks are killed. Can be very deadly in combination with the "Flower" special, so kill them ASAP.


Phase 2 special:
Parasites:
The Nightmare throws a parasite onto random players. After it has grown, it will burst out of the player and deal damage to them, and will then begin healing The Nightmare until killed. If the parasite is on you, drink a sip of your sanfew serum, that will negate its damage.


Phase 3 specials:
Surge:
Nightmare will randomly teleport to a wall on the east or west side of the room - dashing to the other side and dealing up to 75 damage to everyone in its way. To dodge this simply run north or south.
IF you get hit, dont panic - use your combo food and look out for its next attack. Surges can occur back to back.


Spores:
Puffshroom like objects spawn around the arena. Standing within 1 tile of one will cause it to explode, your run is disabled and your attack speed is lowered by 1 tick for a short duration.


Pillars and Sleepwalkers:
After every phase 4 pillars will spawn in every corner of the room, which you have to kill asap. (Special and normal attacks will still hitt you)
If you have killed all 4 pillars "sleepwalkers" will spawn in every corner and walk towards nightmare in the middle. You need to kill them ASAP since every sleepwalker that gets throu will increase nightmares special attack damage.
Usefull weapons to quickly kill them:
Scythe of vitur, Chinchompas, Barrages


Tank role:
The person with the highest defensive stats in Equipment details in your team will be classified as the Tank and be focused by Nightmare.
As the TANK you should stand on the opposite side of Nightmare as the rest of your team the entire fight. This will make you take all the melee attacks, and will allow your team to heal up with soulsplit.

Setups and tips:
Nightmare has ALOT of defence, especially on phase 2 and 3. So everyone should bring a defence lowering weapon to make your fights alot easier.
Best weapons to use: Statius's warhammer > Dragon warhammer > Brackish blade > Dark sunspear > Bandos godsword (or)
You also should bring either a Grand ring (i) or a Ring of vigour, those 2 rings will save you 10% special attack on each special attack, which will lead towards more specs and easier kills overall.

Nightmare is weak to CRUSH so you have to adjust your equipment towards it, hitting it with other styles is realy hard and NOT recommendet.
Best weapons to use: Inquisitor mace > Darksunspear (crush option) > Scythe of vitur (crush option) > Brimstone hammer > Darksire bludgeon > Ogre club (with boots) 

Blood amulet of fury. Since you cant 100% protect against nightmares attacks, a good source of passive healing would be the Blood fury.
Its also very helpful on the "pillar" stage, since you cant use soulsplit against pillars. 

Fighting Nightmare in a 5man team is currently the most efficent way, since you will receive a 15% droprate bonus for beeing the DMG mvp, and a 5% droprate bonus if you took the most melee DMG.

Example setup:

This is currently one of the strongest setups against Nightmare.
Scythe+blood fury - switch is for pillars
Collection pendant 4 -  in the bottom right is for more droprate bonus
Cape of myths (i) -  good way for stat boosting after you brewed down (any other maxcape or even a combat potion will work)
Karambwan - good combo food, perfect for quick healing
Saradomin mixes/brews - good combo food, perfect for quick healing
Sanfew serum - needed for the 2nd phase special attack + restores pray
Molten boots - since nightmare is extremely weak to crush we take all the extra crush bonus we can get

Extra note: You can't use pets for the fight.

To see alternatives for every gear slot check out my Equipment guide:
Night terror:
After completing 100 Nightmare kills you have acces to the stronger and way more dangerous variant of Nightmare the: Night terror

Whats special about the Night terror:
Night terror will deal more damage than nightmare and it also has more hp and defence.
If you die during the fight you will loose all but one item. (You will always keep your most expensive item (first item shown in: "items kept on death"))
You will also keep all your untradeable items - but your untradeables in the death coffer will be alot more expensive.

Night terror has 3 special drops ontop of the normal Nightmare drop table (also lower droprates):
Night terror pet
Inquisitor blessing
Terror orb

Example setup:

Items RISKED in this setup: Dragon warhammer and Efficency blessing (Inquisitor mace will be my +1 save item)

Full melee void will be your best option because of the set effect for this fight.(5% dmg and accuracy boost)
However- if youre using a setup with a Darksunspear you should drop void and take a Lava slayer helm, Fighter torso (e), K3 gloves and dyed samurai legs.




Special Thanks to: @My hero @Triiident for helping me gather footage for this guide and @Roguemx for a final check