Apollyons are created when one Pure-blood and one Half-blood have a child. This is one reason The Gods demanded to the Pure-bloods that Pures and Halfs never consummate with each other.
Alexandria Andros is one of the current apollyons, with her counterpart Seth. Ethos Krain was the first apollyon. Solaris was born in 1611 AD, and was a second apollyon born to her generation. It is rare for two apollyons to be born in the same generation, it has only happened twice.
They have 'marks' (tattoos) on their bodies that appear when they are emotional. Most of the time, others can not see these marks.
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.