Complex sentence: The High Sparrow decides to grant the "Mother's Mercy" and let Cersei return to the Red Keep, but orders that she must return to face trial for the other sins she has not confessed to, and must undergo an "atonement".
Split into simpler sentences: The High Sparrow decides to grant the "Mother's Mercy" and let Cersei return to the Red Keep. However, she must return to face trial for the other sins she has not confessed to, and must undergo an "atonement".