An original anime. And that is the downfall of this particular OVA, being original and having no framework to rely on when going forward with this story. It is basically supposed to be a hero's journey type piece where the boy is given a quest by his dying father to set things right in the world. What it becomes is an acid-induced nightmarish cluster F of different ideas from the story writer. It's first like, let's make a fantasy world with a wide variety of creatures. Then how about we throw in some mecha, yeah mecha robots shooting lasers. Oh, lets go back and make the environment dangerousby throwing in a sea that is dangerous to cross, but we'll add a giant in there as well.
Oh, lets put some faeries in there in an imaginary realm where wishes come true.
Let's go to science fiction next with an ancient advanced city that is in a state of disrepair.
But now, lets escape all that and the whole time we've been in a dimensional plane created by a god like being we now have to destroy....
Yeah, it is just a jumble of ideas mish-mashed together and a poorly written mish-mash at that.