Learn Dutch: Het kasteel is uitgestorven en er gebeurt helemaal niets. - The castle is deserted and absolutely nothing happens.

Niets simply means "nothing". Adding helemaal amplifies the negation, turning it into "absolutely nothing" or "nothing at all". It strongly emphasizes the complete lack of activity in the castle.

Doesn't uitgestorven normally mean "extinct"?

Yes, it literally means "died out" and is used for extinct animals. However, it is also a very common way to describe a place that is completely deserted or "dead quiet", which is why it perfectly describes this empty castle.

Why do we need er before gebeurt?

Dutch often uses er (there) as a placeholder subject when the real subject is vague or indefinite, like helemaal niets. While English says "absolutely nothing happens", Dutch prefers the structure "there happens absolutely nothing" (er gebeurt helemaal niets).

Does helemaal niets always go at the end of the clause?

In a standard sentence structure with er, yes. If you wanted to place heavy emphasis on the fact that nothing is happening, you could move it to the front. Because the verb must stay in the second position, it would become: Helemaal niets gebeurt er. However, the original sentence is much more natural and common.