|Vaguely symbolic of a macro.|
This macro will cast whichever talent you have specced, and even updates the icon and tooltip (albeit with a short delay). Beware that you must name the macro L90 for that last feature to work. Cascade will also target a player, preferring your focus's target (I focus the boss - so it hits a tank), or your target if the focus target doesn't exist and your target is a player, or yourself otherwise.
For shadow priests, replace the third line with:
This will hit your focus (if you have one), otherwise your target if you have one and it's an enemy, otherwise "the boss". Although I think most shadow priests avoid Cascade.