http://www.halopedia.org/Ur-Didact (Not to be confused with the Iso-Didact who fired the rings)
Unknown to the UNSC the Didact survived his fall into the slipspace rift. Hurtling through the portal along with a number of his Promethean constructs, he crash-landed on Installation 03 near the Composer's original location, the Composer's Abyss. Shortly after his arrival he was discovered by Spartan Black Team, whom he killed immediately upon regaining consciousness. The Didact was soon confronted by 859 Static Carillon, the monitor of the Composer's Forge and the keeper of the Composers, who had arrived on the Halo to investigate the recent activation of a service portal leading to its installation. The Didact asked the monitor to take him to the Composer's Forge, where he established a base in a deserted Forerunner city. He requested Static Carillon to deliver him a slightly damaged Halo installation, which he planned to repair and use as his final instrument of vengeance against humanity in spite of his earlier opposition to the Halos. In return the Didact told the monitor he would cease his creation of Promethean Knights from composed humans and that he would not bring them to the Composer's Forge, as Static Carillon considered the constructs abominations.
The following day, on July 26, Static Carillon informed the Didact that a Spartan team led by John-117 had arrived at the Composer's Abyss and had crossed the portal. However, the Didact considered the Spartans' actions inconsequential and decided that he would welcome them to "his world". The Didact soon confronted the Spartans, who opened fire; The Didact then summoned a large number of newly created Promethean constructs to his location to engage the Spartans. While Static Carillon fulfilled its end of the bargain, transitioning Installation 03 over the Composer's Forge, it also turned on the Didact as he had violated their agreement by bringing Prometheans to its installation. The Didact departed for the portal, planning to repair the Halo and use it against Earth; he also took one of the Composers from the Forge and transported it to the Halo. The Spartans followed in close pursuit, assisted by the betrayed monitor.
Back on the surface of Installation 03, the Didact confronted Blue Team once more, intending to finish them off once and for all. However, John-117 managed to stab him in the eye with a combat knife in a surprise attack. Bemoaning his refusal to kill the Spartan when he had the chance, the Didact lifted the Master Chief in the air and crushed the Spartan's helmet. The Promethean quickly halted the other Spartans' efforts at saving their comrade and knocked them all to the ground. Before he could finish them off Static Carillon attacked him from behind and teleported him to the Halo's Control Room.
John-117 soon arrived in the Control Room with the ring's Activation Index and confronted the Didact; the Spartan explained that because the Didact could not be killed by conventional means a different approach must be taken. The Didact became puzzled when John inserted the Index into the Core; he asked why the Spartan would activate the ring just to destroy him. John informed the Didact that he did not activate the ring's weapon but instead disabled its safety protocols; this allowed the monitor to eject a large section of the ring containing the Control Room toward the Composer's Forge. The monitor rescued the Spartan while the Didact was subjected to the Composers' effects, disintegrating him but storing his essence in digital form; John later told Fleet Admiral Terrence Hood that he believed the Forerunner was merely "contained", not dead.