Complex sentence: While hunting, he spotted a deer or stag and started chasing it up a small hill but when he reached the top, the deer or stag vanished but he came to a very large rock and decided to climb it.
Split into simpler sentences: While hunting, he spotted a deer or stag and started chasing it up a small hill but, when he reached the top, the deer or stag vanished. He then came to a very large rock and decided to climb it.