Learn Italian: Chi fondò l'impero? - Who founded the empire?

It marks the third-person singular of the passato remoto (historical past) for the regular -are verb fondare (to found). All regular -are verbs take this accented -ò ending in the third-person singular (e.g., parlò, cantò). The accent is mandatory: without it, fondo is the first-person present ("I found") or a noun meaning "bottom".

Why use the passato remoto (fondò) here instead of the passato prossimo (ha fondato)?

The passato remoto is used for events that happened in the distant past and are completely finished, with no direct link to the present. The founding of an empire is a classic historical event, so you will almost always see it written in the passato remoto in history books, literature, and storytelling.

If multiple people might have founded the empire, why do we use the singular fondò instead of the plural fondarono?

When the interrogative pronoun chi (who) acts as the subject of a question, it always takes a third-person singular verb, even if you suspect the answer will be a plural group of people. This works just like in English, where we ask "Who is coming?" rather than "Who are coming?".

Is impero a masculine or feminine noun?