Complex sentence: The band was initially called "La Patera" but as a band with the same name already existed they adopted the name Marea, that was supposed th be the title of their first album that they instead named "La Patera".
Split into simpler sentences: The band was supposed to be called "La Patera" but that was impossible because a band with the same name already existed. They adopted the name Marea, and their first album was named "La Patera".