Learn Finnish: Vastarinta on niin säälittävä, että maailmanherruus on vihdoin meidän. - The resistance is so pathetic that world domination is finally ours.

It is the present active participle (the -va / -vä form) of the verb sääliä (to feel pity). When used as an adjective, it literally means "pity-inducing" or "pitiful", which perfectly translates to "pathetic" in this context.

What is the anatomy of the compound word maailmanherruus?

It is a compound of maailman (world's) and herruus (mastery or domination). Herruus is formed by taking the noun herra (lord or master) and adding the suffix -uus, which turns it into an abstract state or concept.

Why is meidän used on its own at the end, without a possessive suffix?

In this sentence, meidän is used as a standalone predicate meaning "ours" (as in, "is finally ours"). You only add a possessive suffix to a word if there is a possessed noun directly attached to it, such as in meidän maailmanherruutemme (our world domination).

Are there any special rules for the niin... että... (so... that...) structure in Finnish?

It works exactly like it does in English! The only major difference to remember is the punctuation: Finnish strictly requires a comma before the subordinate clause starting with että.