Like all lore and story in Elden Ring, Volcano Manor and Tanith's background are not so easily discoverable to the player. Fans are slowly piecing the lore together through collected items and sharing it on the internet. To understand Tanith and the manor, fans must also understand the story of Rykard all the way back to before he became the serpent of blasphemy.
Volcano Manor was built on Mt. Gelmir because the demigod Rykard, the oldest son of Radagon and Rennala, wanted to research new forms of magic using the volcano's magma. Rykard was a powerful sorcerer and became known for his disloyalty to the Golden Order and the Greater Will. In fact, he helped his sister Ranni defy her destiny and then proceeded to kill everyone who remained loyal to the Erdtree and Golden Order. Along with him, Volcano Manor was ruled by Tanith, who originally was a dancer that Rykard met and fell in love with in his travels.
However, the demigod and his wife were not alone in the manor. There was an immortal god serpent named Eiglay, and Rykard was charmed by it. Apparently, there was a lost heretical religion that worshipped Eiglay long ago and Rykard decided to use Volcano Manor as grounds for the religion to be reborn. This religion, however, required a ton of sacrifice, so Volcano Manor became a place of torture and death. Eventually, Rykard sacrificed himself to the serpent for immortality and the two beings became one. This lost the loyalty of Rykard's men, who now only saw their lord as a monster. The only one to remain loyal was Tanith herself.
Tanith is the woman that the Iron Maiden monsters' designs are based on. The Iron Maidens were made at Volcano Manor as devices to make more sacrifices for Eiglay, and they would also be used to fight against the Golden Order that sought to destroy the manor. On the roads up to the manor, Rykard and Tanith's forces displayed bodies to scare away any others that dared attack the manor. Tanith and Rykard also decided that they would kill any Tarnished that could rise as an Elden Lord to continue the status quo of the Erdtree and Golden Order. What Tarnished they did not kill, they would use to hunt and kill their own kind as assassins.
According to the games' lore, Tanith loved Rykard all the more after he became one with the serpent. After his death by the player, Tanith goes to his corpse to devour his remains. It is unknown exactly why she does this, as it can be due to emotional devotion or some hope that his flesh can be reborn through her.
Tanith does have a daughter she adopted who works as a scout for the manor. This daughter is Rya, and she has both a snake and a human form. Rya discovers through her questline that she was born from a ritual, likely one that Tanith and Rykard had a hand in, or from the religion of Eiglay that they aspired to rebuild. Tanith deeply cares for Rya, enough to give her a potion of forgetfulness if her origins cause her depression or agony.
Elden Ring is available now for PC, PS4, PS5, Xbox One, and Xbox Series X.