Learn Italian: L'imperatore fondò un grandissimo impero.

It is the passato remoto (historical past) of the regular verb fondare. For the third-person singular (he/she/it) of -are verbs in this tense, you simply drop the -are ending and add an -ò with a grave accent.

Why is the accent mark on fondò so important?

The written accent is mandatory because it shifts the spoken stress to the final syllable. Without the accent, fondo is stressed on the first syllable and means either I found in the present tense, or bottom as a noun.

Why does the sentence use grandissimo instead of molto grande?

Both mean very big or huge, but adding the -issimo suffix (the absolute superlative) is often preferred in Italian to sound stronger and more emphatic. It acts just like a regular adjective, taking the masculine singular -o ending to match impero.

A quick refresher: why is it un grandissimo impero instead of uno?

The indefinite article always matches the sound of the word immediately following it. Even though the noun impero starts with a vowel, the word right after the article is the adjective grandissimo. Since grandissimo starts with a simple consonant, it takes the standard masculine article un.