Here is a Explanation about Demons from the LN ( I took out anything that might be Spoilerish):
Demons or Daemons are Monsters that are Spiritual Lifeforms similar to Spirits or Angels. They do not have a body normally, as they exist in the Spiritual World where they mostly wander aimlessly around. By default, most of the Demons do not have names or any sort sense of self. There are a few exceptions which include those that have developed an ego after being summoned several times and having contact with the Material World.
Within the Spirit World, Demons gain strength and prestige almost entirely by age. The older a Demon the stronger it is. However, it takes a long time to get truly strong within the Spirit World, since Demons are bound by a Magicule limit that forces them to polish their proficiency and efficiency instead unless they reach an age of over 3,000 years.
Within the Material World, as long as a body to possess is available, additional options to get stronger open up for Demons, but the "common sense" within the Western Nations is that Demons can only manifest as Greater Demons at best - with Arch Demons only being thought to be possible when a Greater Demon is named - and an Arch Demon being considered the strongest variation with the sole exception of a certain Demon Lord, who reached the Demon Lord stage of Demon evolution. Of course, that isn't entirely correct since Primordials and more than 3,000-years old Demons can break through the Magicule limit and become Demon Lord-Class in strength, even without a name. All Demons, in general, are considered to have strong resistance against Magic.
Summoning
Demons can be summoned from the Spirit World to the Material World. Demons are usually serious and respect contracts, though that might also depend on which color line they belong to.
The strength of the summoner is important as when the summoner is too weak in the view of the Demon, then the Demon might turn on their summoner.
Once in the Material World first an Ego arises. Each summoner provides more information to the Demon allowing them to become more proficient and knowledgeable with each summoning.
However, a Demon needs a body to stay in the Material World, as otherwise, the Material World will reject the Demon.
Mathias
2020-11-07 19:19:00 +0000 UTC
Sooo.. if i get it right.. that ''archdemon'' killed one silver wing.. second silver wing perform a demon ritual.. called that guy who kill the archdemon but he lost his.. ''life essence'' ? and vanished and then Rimuru summoned him again at the end of episode 23 ?? :D and what ? he gave him the name and now have enough essence he will live.. for quite some time now ? :D also would like to watch the ovas with you :D if they are any good.. i mean.. first ova (after recap) is like only fan service episode but second one should be. interesting :D