DMs, from the unending archives that is DnD, I'm asking for the most twisted, craziest, coolest, or scariest homebrew monsters that your players have ever faced. A little background on the monster to if you don't mind.

Not sure about nightmares, but I created a Daymare. Never got the chance to playtest it, but it's essentially an invisible monster that feeds on fear and attacks creatures in their sleep in an effort to absorb enough energy to become real. Two combats happen simultaneously - one in the waking world and one in the dream world. 

I also once reskinned some Shadows as spirits of beloved NPCs the party had lost and made them trick and then attack the party. It wasn't a difficult encounter mechanically, but boy was it a nasty DM move leading up to a boss fight.

Absolutely beautiful work, it definitely needs to be played. Too good to pass up on and throw in a closet.

I once did a CR17 adaptation of the Mothers of the Terrible Glow from Lord of Light. It was a big bundle of hp with low AC, but it cast sickening radiance as a reaction, corrupted the water it swam through, and had a legendary action which caused even more radiant damage with exhaustion. If you could keep it at range you'd be fine, but get in close and radiation sickness would make for a quick trip to the grave.

Personally I created a CR 11 Mechanical Wyrm that could shift "gears" to better suit it's situation in combat. With three different gears; Piercer, Ripper, and Launcher. Piercer gave it a spiked hull that ups its AC and damages creatures whenever they occupy the same space. Ripper made it's gullet rotate with blades and whenever it swallowed a creature it would also be ripped to shreds in it's throat. Launcher gave it the ability to spray hot oil which also gave the chance for it to throw up swallowed creatures. It hasn't been tested in game yet but I think it's pretty cool.

That is pretty cool. What does it take to change gears, an action? Or can it do that as a part of a multiattack?

It uses it as a bonus action, but if it takes more then 15 points of lightning damage it short circuits itself and reverts to no gear active. I would show you a link to the monster but I don't know how.

**Not my work** but I played a monster that could take a reaction on every turn. It also had a reaction that allowed it to teleport up to 120 feet and make a single weapon attack against anyone casting a healing spell, interrupting the casting and wasting the spell slot (if applicable) on a hit. 

Quote from ThickGatorade >>

DMs, from the unending archives that is DnD, I'm asking for the most twisted, craziest, coolest, or scariest homebrew monsters that your players have ever faced. A little background on the monster to if you don't mind.

 
Mind is Sand Sharks. So my world is homebrew - and there is a land called "The Broken Lands" - which was once lush forest and rivers - and became the epicenter of a war that involved the various planes - and did so much damage that the gods "wiped the slate clean" - in the area - and wiped out the area, turning the once lush forests and rivers, into endless desert lands. However, the magic from the war had an ever lasting effect.

One of them being sharks. They became enchanted and now swim through the sand as easily as sharks would water. They have all the same things sharks do - they can smell a drop of blood for miles, they're fearless, attack until the prey or themselves are killed; while beneath the sand they're under full cover. They can breach the sand (leap) and try to pull people into the sand and begin suffocating them. They have the same thing Sahaugin have - where anyone who isn't full HP they have advantage on explained as "Blood Frenzy."

So every time they're out in the desert and see a fin coming their way, they're like, "Oh damn it."