This is a 5e conversion of a monster originally published by WotC for Dungeons & Dragons 3.5 edition.
Back in 2005 Wizards of the Coast published another great book in their series of “Environment” hard cover supplements for D&D 3.5 called Stormwrack. As you might be able to guess from the title and cover, it’s all about aquatic adventuring. Recently I needed an aquatic monster to pit my PCs against in Curse of Strahd, and wanted something that would seem otherworldly and horrific, without being overpowering (thus, no Aboleth!). I didn’t find what I wanted in the 5th edition Monster Manual, but while flipping through Stormwrack I came across the perfect creature, The Caller From The Deeps
The Caller From The Deeps is an aquatic elemental that has come together spontaneously from the remnants of death that have filtered down into the depths of the seas from above. It becomes sentient, and seeks out life to feed its Neutral Evil hunger. Seemed perfect for Curse of Strahd.
You can see the stats for the original Caller From The Deeps care of The Creature Chronicle: http://thecreaturechronicle.tumblr.com/post/56228293275/caller-from-the-deeps. It is described in Stormwrack on page 140 as…
“an inky blot of cold black water. Closer inspection reveals a mass of liquid tentacles with no central body that writhe and seethe in the water.”
This is my first monster conversion to 5e, and from what I’ve read online, and in the 5e DMG, converting monsters is a bit of an artform, so if you feel I’ve got it wrong anywhere, please let me know. In addition to updating the stats for 5e, I also decided to give it lifedrain to steal hitpoints, and gave the Caller servants called The Drowned that it calls with it’s Summon ability (in the 3.5 version it can summon a medium water elemental or fiendish shark). Since my PCs would be encountering this monster in Barovia, I wanted to make it a bit more thematic. The Drowned are the victims of the Caller, their life sucked from them, turning them into undead. They are based on the 5e Zombie, but rather than using a Slam attack against foes, they focus on grappling creatures and dragging them into the water to the Caller.
The Caller From The Deeps is not my creation. It was created by Wizards of the Coast and is copyright 2005 from the book Stormwrack.