From the web enhancement that accompanied Ghostwalk, the 4th-grade paladin spell door to great evil is silly, provides a break from the scheduled events of the campaign (unless the spell is, itself, such a scheduled event), and is a terrific excuse for evil PCs to suddenly have paladins smiting them.
In essence, it's greater teleport, except that you don't pick the destination, and it offers no save or spell resistance; instead, the single target (yourself or a willing paladin touched) is sent to the nearest level-appropriate evil foe(s) for an encounter where combat is an acceptable course of action. It does not provide a means of return, but it does provide loose navigation information (in the way of distance and direction).