The term Ascension was a religious belief shared by the followers of the Brotherhood of Nod.
The actions of Kane were set to bring about Ascension to the faithful and those that shared his vision of a Tiberium future for the world. After his "death" in the early part of the Third Tiberium War, Kilian Qatar stated that the Brotherhood would need to put all word of Ascension aside while Nod attempted to survive.
Kane devised a Tiberium Control Network from the Tacitus, which would harness the collected Tiberium from the nodes to activate a portal inside Threshold 19. He also developed optical implants along with Dr Johann Pascal, and implanted them in five humans to activate the portal once the TCN was brought online. However the Nod Separatist leader Gideon found out about the implants as part of Kane's plans to Ascend and killed four of the implant bearers during the Fourth Tiberium War, jeopardizing Kane's Ascension plans.
At last, at the end of the so-called "Ascension Conflict", the TCN was online and ready for a direct link to Threshold 19. The last remaining Optical Implant bearer, GDI's Commander Parker, was then brought to the portal in the tower to activate it. He blacks out after Col. James' intervention, paving the way for Kane and some of his loyal Nod followers to Ascend, fulfilling his destiny and bringing about the end of the Tiberium scourge, which Gideon regarded as "the real Messiah".
The fate of the portal after the Fourth Tiberium War is unknown.