There are two kinds of monsters that work well in D&D: The fantasy monster, and the science-fiction monster.
The fantasy monster works because it is familiar. It reminds us of the stories we read when we were children, of folktales passed down from ancient times. A red dragon is a fantasy monster. A vampire is a fantasy monster. An elf is a fantasy monster. Every fact about these monsters should seem like it's passed on from ancient tales. When the Monster Manual says a Rakshasa can be slain only by an arrow that has been blessed by a Brahmin, it seems true--even if it was made up for the Kolchak episode.
The sci-fi monster, though, works because it is strange. It is a creature of horror or wonder that we have never before seen. We fear a dragon because we know exactly what it can do. But we fear a Mind flayer because it is an unkown. These creatures have strange visages, strange habits, and strength and weaknesses that we cannot expect.
Your monster sits in an awkward place between the two. It doesn't work as a fantasy monster, because it is too strange. We perhaps recognize the elements of dragon and vampire and giant, but we do not expect dragons and vampires and giants to be united into one creature. We do not expect giants to transform with the sun. We do not expect dragons to have probosces.
But it also doesn't work as a sci-fi monster, because it is too recognizable. We still see those recognizable elements. The sci-fi creature is the child of a warped and powerful imagination, and when we see those familiar elements, it seems like a failure of imagination.
So my advice is to pick one side or the others. What you have is a good story (benevolent entities are secretly horrible monsters) that will work with either a fantasy monster or a sci-fi monster. If you choose fantasy, then pick an existing monster (such as a werewolf or a pureblood yuan ti) and go with that. If you choose sci-fi, then take your existing monster and tweak the design so we no longer see the dragon or the vampire or the giant quite so clearly.