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.
In Special Region standard, The Apostles are considered to be extremely danger since Rory Mercury and Giselle can tank the entire army of the Empire with little difficulty. However, for modern military standard, they are outmatched against heavy weapons like 50 cal machine gun or explosive weapons since these weapons are strong enough to blow them to pieces to effectively incapacitate them. As the result, most Apostles don't dare to provoke fight with the JSDF, despite they can give modern military some hard times with their superhuman combat power and regeneration ability like in the case of Rory Mercury doesn't want to mess with the whole JSDF and willing to work as MP under them or Giselle ran away from JSDF's bombardment on her offspring dragons.
Despite their obviously superior power over normal human, a lot of people in the Empire still underestimates them due to their arrogant and delusion. The prime example is Zorzal and his pro-war supporters since they never take the fact that Rory and Giselle support to the JSDF as serious matter or Zorzal stupidly called his guards and Ogre to kill Rory without realizing how idiotic it is. The another example is during the first battle of Italica, Pina states that Rory Mercury alone can defeat the whole army of defender of Italica but the bandits that attacked the city still try to futilely fight her anyway.