Question: which third annual Arrowverse crossover event did "Medusa"  features
{'answer': '"Invasion!"', 'supporting_facts': [['Medusa (Supergirl)', 1], ['Invasion! (Arrowverse)', 0], ['Invasion! (Arrowverse)', 1], ['Invasion! (Arrowverse)', 4], ['Invasion! (Arrowverse)', 6]]}