Complex sentence: The PCs journey 4,000 years into the past to the land of Blackmoor, where they are hired by The Fetch, from the adventure "Temple of the Frog", because the cult introduced in that adventure, the Froggies, has become active once more.
Split into simpler sentences: The PCs journey 4,000 years into the past to the land of Blackmoor. There, they are hired by The Fetch, previously seen in the adventure "Temple of the Frog", because the Froggies, a cult introduced in the same adventure, have become active once more.