The effect thereof was resoundingly eternal.

"Thereof" is another (more formal) way of saying "of it".

While you CAN'T say: "I'm sick thereof"

You CAN say: "I don't want to discuss this issue or anything related thereof".

not a perfect example, but you get the gist of it.

(if you were wondering how to use the word "gist", the sentence above is a perfect example).

The Court charges you that the mere fact that there has been an accident and as a result thereof, the plaintiffs have been injured does not, in and of itself, entitle the plaintiffs to recover.

The cast of The Evil Thereof - 1913 includes: Bliss Milford as Mary Randolph - the Shopgirl

It's an adjective.