Learn Finnish: Salaperäinen portaali avautukoon! - Let the mysterious portal open!

This is the 3rd person singular imperative, often called the jussive. To form it, take the infinitive stem of the verb (from avautua, the stem is avautu-) and add the ending -koon (or -köön depending on vowel harmony). It is used for commands or wishes directed at a third party, translating here to Let it open!

Why do we use the jussive -koon form here instead of a normal imperative command?

Standard imperatives are used when speaking directly to someone (2nd person: you). Here, the command is directed at the portaali (a 3rd person subject: it). In Finnish, to say Let it do something or May it do something, you must use this specific 3rd person imperative form.

Why use the verb avautua instead of the more common verb for opening, avata?

Avata is a transitive verb, meaning someone opens something (for example, a person opens the portal). Avautua is an automative verb, meaning something opens by itself. Because the portal is opening on its own in this sentence, we use avautua.

Even though this is a command, why is salaperäinen portaali in the basic nominative case instead of an object case?

In normal imperative sentences, objects often follow special rules (like dropping the final -n). However, salaperäinen portaali is the subject of this sentence, not the object. Because the portal is doing the action of opening itself, it takes the standard nominative case.