Complex sentence: It was found as a pup inside Conganchnes's burial mound, and would only let Celtchar handle it, until one day it escaped, and became a menace to the cattle and sheep of Ulster.
Split into simpler sentences: It was found as a pup inside Conganchnes's burial mound, and would only let Celtchar handle it. One day it escaped, and became a menace to the cattle and sheep of Ulster.