tbh i think the story might be in the hh artbooks...
basically by the time the emperor left back for terror he had discovered and walked the secret web ways of the eldar, when he was building his mighty imperial palace he had a gargantuan dungeon construced and inside it a massive portal into the webways was created. Large enough to allow titans to walk through, the Emperor dreamt of freeing man from his reliance on warp travel and astrotelepathy. Unfortuantly the portal and the passageway beyond required constant maintenance.
initially it required a small portion of the Emperors psychic might but as time went on the warp powers of Chaos grew and threaten to come forth on to Terra via the portal. The Emperor withdrew from the crusade to keep a constant vigil on the portal, he created a massive machine to channel his supreme psychic might into maintain the portal, aka \'The Golden Throne\'
When Horus attacked Terra he left the vigil to Malcador whilst he fought Horus. #
Of course the Emperor was mortally wounded and was brought back to the golden throne where Rogal Dorn forever encased him within the machine so he could forever keep back the monsters of the warp...
The end lol....