Learn Portuguese (Portugal): Na selva escura espreita o caçador.

This is an example of locative inversion. Because the sentence starts with a phrase describing a location (Na selva escura), the verb (espreita) and the subject (o caçador) switch places. This creates a more literary or dramatic effect, drawing attention to the location first and revealing the subject at the end.

Is it mandatory to invert the subject and verb after a location phrase like Na selva escura?

No, it is not strictly mandatory, but it is highly natural and common in descriptive or narrative Portuguese. You could technically say Na selva escura o caçador espreita, but placing the verb before the subject flows much better stylistically and adds a storytelling quality to the sentence.

Does espreitar simply mean "to peek" in this context?

While espreitar commonly translates to "to peek" or "to take a quick look", in the context of a hunter in a dark jungle, it carries the heavier nuance of "to lurk", "to spy", or "to lie in wait". It implies watching secretly and attentively without being seen.

Could the adjective escura be placed before selva?

Yes, you could say Na escura selva, but placing the adjective before the noun makes it much more poetic. It would emphasize the darkness as an inherent, emotional quality of the jungle. Keeping it after the noun (selva escura) is the standard way to state a factual description.