Complex sentence: Athelstan stays behind with King Horik and is captured by Wessex's soldiers and crucified as an apostate, but is spared by the mercy of King Ecbert.
Split into simpler sentences: Athelstan stays behind with King Horik and after being captured by Wessex's soldiers he is condemned to death as an apostate. At the last moment, he is spared by the mercy of King Ecbert.