Learn French: Cette aventure est plus violente que la première. - This adventure is more violent than the first one.

Adjectives in French must match the gender of the noun they describe, even when used in a comparison. Because aventure (adventure) is a feminine word, the adjective takes the feminine form violente instead of the masculine violent.

Why do we say Cette aventure instead of Ce aventure?

Cette is the feminine form of the demonstrative adjective, meaning "this" or "that". Since aventure is feminine, we must use cette. If it were a masculine word, we would use ce (or cet before a vowel).

The English translation says "the first one". Why is there no word for "one" in the French sentence?

In French, you rarely need a translation for "one" when referring back to a noun. You just use the article and the adjective. Here, la première literally means "the first", which perfectly translates to "the first one". We use the feminine forms la and première to match the feminine noun aventure.