Complex sentence: The pronounciation of this Doom was the immediate cause for (already reluctant) Finarfin and his followers' return to Valinor, where they asked and received the forgiveness of the Valar.
Split into simpler sentences: The pronounciation of this Doom was the immediate cause for the return to Valinor of (already reluctant to proceed) Finarfin and his followers. There they asked and received the forgiveness of the Valar.