The Dark is the sister card to The Light. The Dark's main abilities are being able to warp space and neutralize non-magical beings after a certain amount of time. It is also able to bring about night. This is one of the last cards to be sealed, along with The Light. The Dark has special sealing conditions; Sakura had to realize which card it was and then she must find a way to break the darkness.
The Dark appears in Episode 42, where it's darkness interupts Sakura's and Syaoran's class play just as they were about to kiss. The darkness engulfed everyone except Sakura and Kero, incapacitating everyone else in the area. Sakura was able to beat The Dark with The Light, which had been in Sakura's soul ever since she opened the Book of Clow. This was all apparently a test and both cards wished to be sealed together since dark and light were always together, and thus, always sealed together.