Actually, if you read about the Arraxian Crown in the World of Etheria section of the site, it implies that Adramelech is the demon king.
"The Arraxian Crown is by lead three demons sent from Infernia by Adremelech to personally oversee his return to Etheria."
This appears to suggest that Adremelech has the authority to at least command high-ranking demons and that he once was in/ruled Etheria. So this could mean one of two things:
1. Abaddon could be one of the three demons which he sent to Etheria. If so, the "other demons" the flavor text refers to could be the other two demons Adramelech sent to Etheria. He might just be the biggest baddie of the three.
2. Abaddon inhabits Infernia and is just some random demon. Abaddon is a pretty popular name for a demon in games in general, so I'm not surprised to see it here.
However, the main reason Adramelech is probably the king of the demons is because his name roughly translated from Hebrew/Arabic literally translates into "King of the Rock," or probably more accurately, "King of Hell" or "Brutal King."
From my voracious reading habits, I knew that Melech meant king in Hebrew, but I didn't know what Adra meant. Looked it up, and it means "rock" or "hard."
Suffice to say, whoever made the card must be familiar with ancient languages, or at least looked up cool words to make the name.