A Ceremony to Call for Divine Help is when an individual is able to perform a ritual to ask a god for help, which they must obey.
In order to perform the ceremony, one must take the dying breath of a god and go to a temple of shrine dedicated to the deity they wish to summon. When they release the dying breath and recite the enchantment, the deity will come. The prayer "must be uttered through through the rainbow" or rather an Iris Message which calls upon Iris the goddess of the rainbow to deliver it.
However it is possible for a delayed reaction to occur with the god or goddess called does not immediately come.
Apollo performs the ceremony using the dying breath of Harpocrates to call his sister to help defend Camp Jupiter during the Battle of San Francisco Bay to help the Twelfth Legion Fulminata defend itself against the forces of Triumvirate Holdings. However there is a delay in the completion of the ceremony and the goddess of the hunt's arrival, when she arrives she kills Tarquin and cures her brother of his zombiefication.