Learn Swedish: Den rikaste mästaren vill slå rekordet. - The richest champion wants to beat the record.

Because it is in the definite form. When you use the standalone definite article den (the) before a superlative adjective, you add an -e to the end of the -ast ending. That is why rikast (richest) becomes den rikaste (the richest).

Why do we have both den and the -en ending on mästaren?

This is a reminder of Swedish double definiteness. When you put an adjective in front of a definite noun, you must use the free-standing article den (for en-words) AND keep the definite suffix on the noun itself, which is the -en in mästaren.

Does slå mean to beat or to hit?

Slå literally means to hit or to strike, but Swedish uses the exact same concept as English when it comes to records. The phrase slå rekord means to beat a record or to break a record.

Why isn't there an att (to) between vill and slå?

Vill (wants) is a modal verb in Swedish. Just like can or must in English, modal verbs are followed directly by the infinitive form of the next verb without using att (to). You simply say vill slå.