Learn French: Le dieu de ce royaume fut très puissant. - The god of this kingdom was very powerful.

fut is the passé simple (simple past tense) form of être (to be) for the pronouns il, elle, and on. The passé simple is highly irregular for common verbs. You will mostly encounter it in reading, like in novels, fairy tales, or historical texts, where it takes the place of the passé composé (like a été).

If fut means "was", why not just use the imparfait (était) here?

You would absolutely use était in everyday spoken French! However, in literature or historical writing, French uses the passé simple (fut) for completed past facts or actions. Using fut here gives the sentence a formal, mythical, or historical feel—like a narrator telling an old legend about a god whose era is completely finished.

Why is it ce royaume and not cet royaume?

royaume is a masculine noun starting with a consonant, so it takes the standard masculine word for "this" or "that", which is ce. As a quick reminder, you only use cet before masculine words that start with a vowel or a mute H (like cet animal or cet homme) so that the words flow together easily.

Why isn't dieu capitalized here?