Complex sentence: Grant had received a letter from Catesby inviting him to a meeting that took place in January at the Catherine Wheel inn at Oxford, where he and Robert Wintour swore an oath, after which they were told of the plan.
Split into simpler sentences: Grant had received a letter from Catesby inviting him to a meeting that took place in January at the Catherine Wheel inn in Oxford. There, he and Robert Wintour took an oath, after which they were told of the plan.