When a Human dies, if they are filled with despair they lose their heart and become a Hollow. When hundreds of Hollow join together, they form a Gillian. Should a single soul within that Hollow become aware, they take dominance over the Gillian. Gillians then begin to consume other Hollows. After consuming so many, they evolve into an adjuchas. As an adjuchas consumes hundreds more Hollow, they will eventually become a Vasto Lorde. With intervention by the Hogyoku or very rarely naturally, they can become an Arrancar. The top 10 strongest Arrancar are the Espada.