The infernal one looks like a 5 meter tall Rakata, the species who lead the Infinite Empire 20,000 years prior to KOTOR, who also built the star forge.
I THINK the OP stated that the Infernal One was somesort of minor deity among the Rakata. Dunno.
Yeup, the Infernal One is Rakata. The Eternity Vault was a Rakata prison.
I'd probably be cranky too if the warden left me to rot for 20,000 years.