Nightmare
Nightmare is a melee support jungler (or roaming support) who focuses on Ambushes, Vision Denial, and Disruption. It is very good at picking people off and is among the best teamfighters in the game. While it isn't tanky, and it doesn't do much damage itself, it specializes in turning the enemies strength against them with its ultimate.
Nightmares model is seen differently by each champion, based on what that champion most fears.
Ex. Nautilus sees Nightmare as a Hand reaching out of the ground, made of the dark material that dragged him into the depths. Aurelion Sol sees Nightmare as a dying star. Draven would see a defeated version of himself. Lux sees the Demacian soldiers who dragged her from her home and forced her into service. Mordekaiser sees a destroyed version of his skull, floating around. Lucian sees his wifes soul being tortured in Thresh's lantern. Jhin sees himself. Spectators see a shadowy ball.
Passive: Jumpscare If Nightmare comes out of the fog of war within 500 units of an enemy, that enemy is feared for 2 seconds. Over the duration of the fear, the victim takes 50 (+5 per level) (10-30% AP) magic damage and is afflicted with nearsight, though they retain vision of Nightmare. The same unit may not be feared more than once every 15 seconds.
Jumpscare is your primary ganking tool; if you can walk out of a bush or around a corner to surprise an enemy, the 2 second fear is more than enough to secure a kill. It is also useful for clearing your jungle; walking around a corner into the jungle creeps line of sight will trigger the fear.
Q: Shadows 20 second cooldown, 100 mana Covers a target area with shadows (75% the size of Trundle Pillar's Slow Zone), lasting 1 minute. Enemies inside take 30/40/50/60/70 (+.2 AP) magic damage per second. Enemies outside the shadows cannot see anything that happens inside the shadows. Attacking units outside the shadows will not cause the attacker to be revealed. Shadows will block vision like natural terrain. Wards inside of Shadows are ineffective.
Nightmare can apply Jumpscare by leaving the shadows, but only if the enemy team did not see him enter the shadows.
If Nightmare is killed, or if enemy champions outnumber allied champions in the shadow, the shadow dissipates after 1 second.
Great for both mindgames and teamfights, Shadows has endless uses. You can set one up randomly to zone people; they don't know if anyone is in there, and they have to facecheck to see if there is. You can set up two in the opening to Baron or Dragon to deny vision in the pit. You can throw one on top of your team, rendering them effectively invisible. You can even just throw one up as an escape, allowing you to fade out.
W: Fade 15 second cooldown, 90 mana passive: Nightmare can see the limits of the enemy teams fog of war. Wards are taken into account. active: After a 1 second channel, teleport to a spot in the enemies fog of war (600/1200/1800/2400/3000 range). Only usable if you are currently in the enemies fog of war.
Fade is very good at four things. First, it helps you skirt sight ranges, which is essential for ganks. Second, it helps you get in position to proc Jumpscare: Fade into a side bush in bot or top, and then have your laner bait the target near, or just warp into a bush in the enemy jungle to cheese them. Third, it is exceptional for rotations; a 3000 range blink on a 15 second cooldown would always be good for that. Fourth, you can use the passive as a pseudo ward scanner (though it will not allow you to kill the wards).
E: Panic 9 second cooldown, 100 mana Shock a target enemy with your presence, dealing 120/140/160/180/200 (+.5 AP) magic damage, fearing them for 1.5 seconds and granting them 50% bonus movement speed while feared. Any unit they collide with will take the same damage and be feared.
Panic is used for 4 purposes. First, it is your only nuke (everything else is a medium damage DOT), so you it is great in a fight. Second, due to its nature, it works very well for clearing the jungle. Third, it is a standard part of your ganking combo; Fade into a nearby bush, Jumpscare the enemy, and then use Panic to send them running away from their tower. Fourth, it is an AOE fear; amazing for teamfights.
R: Your own worst Nightmare 150/120/90 second cooldown, 150 mana Forces all enemies in a target area to fight their own worst enemy; themselves. Summons an apparition of every champion hit, lasting 5 seconds. These apparitions are taunted onto their counterparts, take 200% damage and deal 50/60/70% the damage of the original. Apparition attacks work exactly like the original; applying any passive abilities (ex. Silver Bolts), and item effects (ex. Hurricane) If the original dies, so does its respective apparition. If Nightmare dies, all apparitions die.
As if Nightmare weren't already a good teamfighter with its Shadows and Panic. "Your own worst Nightmare" can potentially turn any teamfight into a 10v5. Is the enemy carry really fed? Well, that Jhin must continue to live in fear, because Nightmare is able to turn his 850 AD against him. It is also incredibly good against champions like Ezreal or Elise, who rely on skillshots that stop after one target, as the Apparition is likely to bodyblock quite a few of those.