A mysterious entity in the Mega Man Battle Network series.
DarkPower is created by NebulaGray. In Mega Man Battle Network 4 MegaMan.EXE gets infected with the DarkPower from handling a DarkChip. When his mood changed to anxious, MegaMan will get 2 strange DarkChips in the chip select screen. While these are insanely powerful, he loses 1 point off of his HP max, and his soul is tainted by darkness a tad bit more. Once he is dark, certain chips become unavailable to use, while others become available. Also, when you use a DarkChip, a strange effect will occur for the duration of the battle, depending on which DarkChip you use.
In Mega Man Battle Network 5, DarkPower infects various other NetNavis, and can even be seen in a physically manifested form. The system works a bit different, up to 3 DarkChips can be placed in your Folder at a time, and they can be used in battle just like normal chips, without long-term side effects. MegaMan can still only use them if he get's anxious, but he can also fuse with them to create a Chaos Soul Union, which allows him to use the chip as his charged attack for one turn. However, his charged attack fluxes randomly, and in one form it uses the DarkChip, in the other form, it releases DarkMegaMan.EXE!