From A Wiki of Ice and Fire
It is believed by the Dothraki that consumption the raw heart of a stallion during pregnancy will make a Khal’s son strong, swift and fearless - but only if the mother can eat it all. If the mother chokes on the blood or retches up the flesh, the omens are less favorable; the child might be stillborn, or come forth weak, deformed, or female. The stallion heart ceremony takes place in the evening in a chalk pit ringed by torches in Vaes Dothrak. When Daenerys Targaryen is pregnant with her son Rhaego she takes part in this ceremony. She manages to consume all of the stallion heart.
Daenerys Targaryen's Stallion Heart Ceremony
With the stallion’s blood up to his elbows Khal Drogo set the steaming heart before her, Dany took it with both hands, lifted it to her mouth, and plunged her teeth into the tough stringy flesh. Dany’s handmaidens had helped her ready herself for the ceremony. Despite her tender mother’s stomach she dined on bowls of half clotted blood to accustom herself to the taste. She starved herself for a day and a night before the ceremony in the hopes that the hunger would help her keep down the raw meat.
The wild stallion’s heart was all muscle and she had to worry it with her teeth and chew each mouthful a long time. The taste threatened to gag her but she made herself chew and swallow. Although her stomach roils and heaves she kept on, her face smeared with the heartsblood that sometimes seemed to explode against her lips. Dany looked at Drogo whenever she felt her strength failing. Towards the end Dany thinks she glimpses a fierce pride in his dark, almond-shaped eyes, but she could not be sure.
When it was over Dany turned her eyes to the old women, the crones of the dosh khaleen and proclaimed in her best Dothraki “Khalakka dothrae mr’anha”! A prince rides inside me! She had practiced the phrase for days with her handmaiden Jhiqui The oldest of the crones, a bent and shriveled stick of a woman with a single black eye, raised her arms on high and shrieked . Khalakka dothrae! The prince is riding! The other women answer “He is riding” , proclaiming “Rakh! Rakh! Rakh haj ! A boy, a boy, a strong boy. The crone with a single eye then delivers the prophecy that Daenerys Targaryen’s son will be the Stallion Who Mounts the World.
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