here's one way
1. make a new map to serve as your intro map.
2. fill it with the white (transparent) tiles to make it black ingame.
3. set a starting point anywhere in the map
4. put none as your character sprite.
5. make an event and put message display options (RMXP - Postality)
6. in the event box, select Window Position Middle, and Don't Show window.
7. start adding some text.
8. if you are done with the prologue, just set the message box options to bottom (or top, whatever it is you want) then Show window. then go to change hero graphic (page 3) in that same event to put your old character graphic back.
this is just one way
another way is to use DubeAlex's Text Scrolling Script
http://www.dubealex.com/asylum/index.php?showtopic=52