Complex sentence: The largest of them is known as The International, which is hosted by Valve and takes place annually at the KeyArena in Seattle, while also sponsoring smaller, but more frequently held tournaments known as the Majors.
Split into simpler sentences: The largest of them is known as The International, which is hosted by Valve and takes place annually at the KeyArena in Seattle. Valve also sponsors smaller, but more frequently held tournaments known as the Majors.