D5 is Bagramon's acronym for his "DIMENSION DELETE and DEADLY DESTRUCTION DAY" plan which he works towards in Digimon Fusion. After gathering all 108 Code Crown fragments, he rearranged the Digital World into seven Lands surrounding his Bagra Pandæmonium, then instated the seven Death Generals to eke out negativity from the Lands' inhabitants, which he poured into the completed Code Crown in order to transform it into the Dark Stone. D5 involved destroying all dimensions before fusing them together using powers of darkness to unify both the human world and Digital World into a new world in Bagramon's image. Bagramon (as DarknessBagramon) stated he intended to remove the imperfections of humans using his powers as well as make the Digital World a much better place.
D5 was the day that Bagramon was building up to, by gathering all 108 Code Crown fragments, he created the Dark Stone, a corrupted Code Crown fueled by the negative energy gathered from the seven countries formed when he reformatted the Digital World. When the Dark Stone was created at first, though, it wasn't corrupted enough, so throughout the second phase of Digimon Fusion, the Death Generals were gathering negative energy, by torturing their citizens, thus fueling the Dark Stone. Once it was fueled enough, Bagramon combined with it to plunge his hand to the human world. He said he was to create a new world by first destroying both worlds and combining them under his rule using the Dark Stone. Everyone tried to stop Bagramon but in the end they failed and it also led to the death of Shoutmon. The effects of the hand on the human world turned everyone to stone, except for Zenjirou and Akari because they were saved by the DigiCards of Examon and WarGreymon. After the creation of DarknessBagramon, he went to the human world to destroy what was left of it. But Taiki stopped him and stalled his army so he could plunge into his body to find Shoutmon's DigiMelody, since a Digimon never dies as long as their data remains and is transported to the Code Crown. Taiki was caught by DarknessBagramon and was tortured in his body by a manifestation of Bagramon's previous form. Then, Beelzemon saved Taiki just as he was about to be blasted by the manifestation of Bagramon, who was stopped by Shoutmon. Soon after, Shoutmon explained to Bagramon that the Code Crown chose Taiki and not him, so all allies who died, were brought back to life to aid them. In the end, the Code Crown gave Taiki and his fellow Generals, the ability to perform Final Xros to form Shoutmon X7 Superior Mode. Shoutmon X7SM destroyed DarknessBagramon with his Final Xros Blade. Finally, everything became normal again and with Shoutmon now the Digimon King, Taiki gave his regards.