Complex sentence: She told him that she was his accuser in heaven, on account of an unchaste thought the (married) narrator had once had concerning her, though only in passing; he was to pray for forgiveness for himself and all his house.
Split into simpler sentences: She told him that she was his accuser in heaven, on account of an unchaste thought the (married) narrator had once had concerning her, though only in passing. He was to pray for forgiveness for himself and all his house.