Learn English – What does “I’m all the wiser” means

That is so wrong it makes my eyeballs bleed. Let's consider some syntactic tests, shall we?

Adjectives can be compared with -er and -est (or more and most):

A bigger house
The biggest house
! More the house
! Most the house

Adjectives can be placed in a predicate:

The house is big.
! House is the.

Adjectives can be coordinated with and:

Red and green house
! The and green house

Conclusion: the is not an adjective. The fact that it modifies a noun does not make it an adjective, because it has none of the grammatical or morphological properties of adjectives.

The above isn't properly punctuated. It should be:

The high-backed chair in front of the fire moved suddenly, and there rose up out of it - like a pantomime demon coming up out of a trapdoor - the alarming form of Uncle Andrew.

There are two statements here:

The high-backed chair in front of the fire moved suddenly.

The alarming form of Uncle Andrew rose up out of [the chair], like a pantomime demon coming up out of a trapdoor.

The demon part is a simile; the author is comparing Uncle Andrew to a monstrous creature; because, I guess, of the sudden way he gets up from his seat.

I think the construction: "There rose up... the form of Uncle Andrew....." is intended to make it sound archaic, like a legend or a storybook.