Twelve Apostles (十二使徒 Jūnishito) are apostles that serves various gods and goddesses. The requirements of becoming a demigod is not clearly defined but becoming an apostle (a type of demigod) is pretty simple; be chosen by a god. All the demigods that showed up in the novel so far are apostles with one exception. The Forest Goddess Yareharun is a sentient forest who became a demigod through sheer evolution alone. She serves no god as her master.
An apostles body regenerates any wound it recieves. Even if their body is torn apart the parts will heal if they are stuck together, in other words they cannot die. It is stated by Giselle, however, that an apostle can be imprisoned or even de-facto eliminated (at least until they ascend) by breaking them into small enough pieces and separating them over large distances, typically by burying said pieces, suggesting they could be destroyed with heavy weapons and explosives, if the parts were separated sufficiently.