Apollyons are created when a Pure-blood female and a Half-blood male have a child. This is the main reason why the births of them are considered rare. This is why the Gods demanded to the Pure-bloods that Pures and Half-Bloods can never consummate with each other.
Alexandria Andros is a former Apollyon. She is later turned into a demigod after Apollo gives her and Aiden ambrosia to live together. It is because of a deal Seth made with the gods. Seth used to be her counterpart/other-half until he was turned into a God in the Titan series. Ethos Krain was the first Apollyon. Solaris was born in 1611 AD, and was the second Apollyon born to her generation. It is rare for two Apollyons to be born in the same generation; it has only happened twice. This is considered a danger to society because it could make one Apollyon drain the power from the other, causing the Apollyon to become a God Killer.
They have 'marks' (tattoos) on their bodies that appear when they are emotional. Most of the time, others can not see these marks. They can also have gold/amber eyes. They are usually created by Apollo, hence the name; though, in Seth's case, he was created because of Ares' diabolical plans.
Apollyons can control the four basic elements; air, fire, water and earth, as well as the fifth element, akasha. They can use compulsion to get what they want; however compulsion is forbidden.
If there are two in one generation, there is a possibility of one becoming a God Killer. This happens when the 'second' Apollyon comes into his/her power (called awakening) at age 18 and the 'first' drains him/her of all their powers.
The marks are as follows: Courage, Strength, Power & Invincibility.
An oracle predicted that there would be one Apollyon per generation. The first Apollyon was named Ethos Krain, born of a pure-blood and a mortal in year 1256 AD. He was the first half-blood to show hematoi (pure-blood) like powers. As foreseen by the oracle, his powers were awakened on his 18th birthday.
Since the birth of Ethos, there has been one Apollyon in every generation, until the tragedy of Solaris, when a second Apollyon was born in one generation. She was discovered in the new world, with no information on her parents or last name. It is believed that the first Apollyon of the generation felt her awaken, and joined her in the new world.