To me, Eldritch carries the connotation of wrongness. The tentacles and nameless horrors are just thematic elements and arc words. Something that is eldritch is wrong, it's a person with a gaping head wound smiling and giving you a cheerful hello wave, or a girl bending over to pick a flower and her knees move the opposite direction from what they should. For a long time, the tentacle was an alien fixture, a boneless writhing thing, something of horror. It's now so prevalent in hentai and anime that it's lost it's shock and horror.
Eldritch is something that you feel in your gut, something you see but you can't believe. It is the supernatural, and hyper sensitivity to the mundane, or a sense of perspective knocked so far out of whack that grains of sands are the size of mountains, or galaxies can be used as sprinkles on a cake.