The Scarlet King was born with the planting of the Tree of Knowledge, called Khahrahk at the time. He was the smallest of his siblings, but the only one aware, and it brought him great pain. He decided that existence itself is painful and that he'd have no part in it, as well as destroying existence itself. He started by consuming his siblings and growing weaker on their essence.
He vowed to destroy the Tree, the Creator and Creation, and consumed or subjugated all the other gods, naming himself "King of the Darkness Below". He declared war on creation itself, one which will not end until the very end of everything. He took Sanna, one of the gods he subjugated, forcibly as his wife and sired seven daughters on her. Sanna died after birthing them, and the King took his seven daughters as his new brides, sealing them so they would not die as their mother had. From his seven bride-daughters he had seven children called Leviathans.
The Foundation was able to neutralise or destroy the first six, but the seventh—the strongest—had yet to be born. It resides in SCP-231 and is prevented from existing by using the Montauk Procedure.
The seventh daughter gave birth to SCP-999 after O5 ordered to stop Procedure 110-Montuak. O5 ordered the stopping of 110-Montuak to see what would happen, the result was SCP-999.
The Scarlet King's daughters are as follows: A'tivik, A'ghor, A'distat, A'zieb, A'nuht, A'tellif, and A'habbat.