back in the alpha version of warcraft, it would change the time of day to 12:00 (midnight) for 1 minute. This would allow the units if from NE (which i think had it) to be under full Shadow Meld (which acted like pernament invisiblily)
I'm pretty sure that Twilight simply changed the time of day to sunset immediately. The ability was on the Priestess of the moon, and did not give any invisibility powers. However, because the sun set, all NE units that had it, could shadow meld.