continuing a brief hijack
to kafkonia and tx7321:
Twilight Calling is a weird little gem of the "M" series of basic adventures. It's the assumed final step before the PC's become immortals themselves, as they are tasked with defeating (in various ways) the immortal rulers of seven demiplanes.
They do this and then venture into the castle pictured on the cover to battle and stop an incursion of reptilian beasties called the carnifex.
There are some problems with the module. The puzzles are cliche' and kind of lame (as in... you've seen them before and they are not challenging.) The castle, despite impressive artwork is pretty much unmapped save for a short maze. There are moments of intense silliness (a trap filled with corrosive barbecue sauce?)
However, the realms of color are mapped beautifully, their rulers have personalities and the encounters are reasonably challenging. Each realm seems to have a powerful dragon you get to fight.
I thought it was the absolute best adventure EVER in high school. This one module went on to be the foundation for 4 long-running campaigns of mine, using the planes of color as my Outer Planes and basing the gods loosely on the original immortal rulers within.
good times.