Greetings @ximena,
Looks like Siofra and Jessica have come to the rescue! 
Apologies that I don’t have too much when it comes to prayers, but adding to resources for you to consider, I know @Amethyst just shared a great book about Dark Goddesses in Book Club that you might find interesting. There’s also @Silverbear’s Morrigan anointing oil recipe and this discussion about all things Morrigan.
Wishing you all the best on your work with the Morrigan- blessed spellwork, Ximena! 