Complex sentence: Foster explains to Ryu that they have been monitoring the activity of someone named Guardia de Mieux--also known as "The Jaquio"--who has moved into an Amazon temple where the body of the demon is located.
Split into simpler sentences: Foster explains to Ryu that they have been monitoring the activity of someone named Guardia de Mieux--also known as "the Jaquio". He tells Ryu that the Jaquio moved into an Amazon temple where the body of the demon is located.