Not just.. tentacles and darkness. Perpetually changing, not cemented in form, with an otherworldly feel to it. Completely unrecognizable by most human descriptions, and only able to be viable perceived by those fine enough to be an adept wordsmith.
Madness string 84048
I don't believe there are subjects that are inappropriate for children. There are inappropriate ways of telling these subjects. So tell them proper. Kids know there are horrors, so tell them the names of the horrors. Tell them these horrors, as with all things, can be fought. Give them the tools, and show them ''how''.