Complex sentence: The Doctor works out a plan to lure the Eater back to the gate during daylight, but requiring someone to prevent the creature from escaping the gate until sunset, but due to the time dilation of the portal, whoever does it will age many years.
Split into simpler sentences: The Doctor works out a plan to lure the Eater back to the gate during daylight, but it requires someone preventing the creature's escape until sunset. However, due to the portal's time dilation, whoever does it will age many years.