Full moon

The moon, fully illuminated from the view of Earth. (TV: Kill the Moon)

You may be looking for the nightclub or the unproduced SJA story.

Full moon referred to when half of the Moon's surface was illuminated by the Sun and could be seen from Earth.

In order to transform into their true form, Lupine Wavelength Haemovariforms used the moonlight from a full moon to gain enough energy for the transformation. However, in contrast, a high concentration of moonlight was also fatal. (TV:Tooth and Claw)