Ever since -632 AR Orcus has used that tarrasque from that time on his own plane in the Abyss as a pet to play with unwanted guests (which are many). This brings Orcus favors (and threats) by his demon lord brethren.
In 666 AR Orcus holds court with a special guest from Golarion, Nex, whom travels back and forth in an effort to set up a temple for Nex's benefactor, Orcus, a powerful demon of the undead.
From 667 through 1890 AR, Orcus battles many battles with other demon lords from their intrusion through his plane to travel to one of a handful of gates leading to Golarion.
Norgorber prays to Orcus after being brought a tableau of ancient writings and symbols of which Norgorber successfully translates "The Last Call to Orcus". This prompts Orcus to secretly bequeath Blasphemousness (the opposite of Sainthood) unto Norgorber to become his new disciple and empowering Orcus as a Demon Lord among his peers and rivals.
In 1891, With Nex and Norgorber, Demon Lord Orcus unleashes "The Feast of Flesh". A thousand souls were driven into Quantium in an attempt to slay Geb (whom Orcus tried to befriend earlier only to have been betrayed because his being in league with Demogorgon, Orcus' chief demon lord rival), whom Geb then slain by his twisted form of death magic. Orcus was on the other end of the death vortex (a "Skeletal Cyclone") which sent the newly dead souls to Orcus wherein he enslaved them and bonded them into demonic skeletons.
In 1892 an Orcussian Cultist Oracle fortells Norgorber becoming a powerful ally to Orcus but at a cost... Norgorber would not be directly recognized as one of The Ascended and his true followers would disband. This was a fallacy and is widely still believed today amongst the other religions. One of Norgorber's names is "The False God" because all that is known about him is falsified. For if the truth were revealed it would mean he would cease to exist and would cease being in power.
1893 Norgorber quietly (among his rivals and most of mankind) becomes the only evil entity to join The Ascended passing the Test of the Starstone.
2490 Orcus births his first of the Incubed, the demon Treerazer, in hopes all that is green and life-supporting is razed from Golarion. Many times Treerazer tries to raze Orcusgate, so finally (at a healthy size and too much for Orcusgate) in 2497 Treerazer was unleashed in Golarion onto one of Orcus' favorite enemies: the elves and their Sovyrian Stone, where Treerazer begins his true destiny of destruction. This causes most of the elves of Golarion to flee to their homeworld.
Later in 2632 elves return en masse back through Sovyrian Stone and attempt to slay Treerazer. It takes a full 666 years to slay Treerazer, where then he returns to Orcus to recuperate and plot the demise of the elves whom tried to slay him.
In 3298 Rovagug awakens from his wounds and free of his bindings... in Orcusgate.