Ok, Im going to begin connecting the prior fiend lords with the Yogsathoth story starting with the chained god as he is called.
The chained god was one of the most cunning of the fiend lords that resided within the layers of Numer, namely in the layer of Mithrask, the breeding pits of Numer. Born to Pilias of the old order, he held high influence over the weaker fiends, controlling a mass army of hideous mindless automatons bent to his will. At this time, Yogsathoth was merely an oracle at the side of the gods of Numer, but secretly he was quite fond of the sly one as he called him. As the wars came and the chained one passed out of Numer and onto earth for the battle. The chained one remained on earth, spreading his evil corruption across the lands, enslaving all he met in honour of the gods of Numer. Secretly though, the chained one decieved them in hopes of amassing his own personal army. The chained one was relieved when the gates of dusk were sealed and destroyed for now he could live out his desire...his desire to be in charge rather than some god. He smirked as the evil gods fell and turned his back on the other fiends and began to hatch his plans for domination. Now, Yogsathoth had become a god due to the divine essences that were left by his former masters. Yogsathoth saw that the chained one betrayed the gods as Yogsathoth betrayed them to by not warning them of there iminent deaths. Secretly, Yogsathoth is proud of the chained one...one after his own heart Yogsathoth likes to say. The first of all the fiends he wishes to control is the chained one but for now all Yogsathoth can do is wait.
Pilias - Pilias was the father of the chained one. Pilias was part of the line of the fallen gods of Numer. Pilias resided in hell along side the gods as an advizer. When the fall of the gods came Pilias was broken...he did not understand how the leader of his line could fall. As Yogsathoth gained power, Pilias soon understood...Yogsathoth lied about his visions and now the dark lords had fallen. Yogsathoth was glad to see someone cunning enough to see his plan. With that, the rest of the fallen gods lines were slain by Yogsathoths command to make way for a new era in Numer. Pilias to was slain.
Ok, for now. Also, if you wish to add fluff and stuff...go ahead.