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