Sínodo is a boss from Blasphemous 2 located in The Severed Tower, an eldritch abomination under the service of the Miracle. Upon defeat, it gives the Key of the Council which opens one of the cages from the Chapel of the Five Doves.
Although not confirmed, the Key of the Council might imply that Sínodo is related to the Basilica of Absent Faces. The item's lore showcases a pontiff staring at the paintings of his predecessors, speaking about "how their faces will one day escape from the paintings" and fuse together as one being. This might explain why the paintings of the Basilica ended up being faceless while Sínodo's vestment had many faces.
Sínodo is ultimately from Greek σύνοδος (súnodos, "meeting, assembly"), whence also English "synod", a Christian council (hence the name of the Key kept by this boss).
Sínodo's vestment resembles a cope, a Christian liturgical garment covering the wearer's shoulders and arms.