One thing that’s scary is to surprise players with the unexpected, and defy and undermine their ability to figure out what’s going on and to control things. Many gamers, especially these days, are used to being coddled. Of course, if some players might respond to some undermining by rage-quitting rather than admitting they can’t handle your game.
There are various ways you can play with that general dynamic… though I’m not familiar with the limits of RPG Maker. But some ideas, some of which may or may not be possible with RPG Maker:
* Have the tension start low but slowly increase, and then when things start to get more tense or threatening, have the fear/tension level change in an unpredictable pattern. But have the maximum fear/tension levels get higher and higher as you go.
* Have them think they know what’s going on, but then have twists that show they were wrong about some things. Then have them be wrong about things they were probably counting on.
* If it’s possible, unexpectedly deprive them of game control/safety mechanics they may have been counting on or taking for granted. Things like maps, the ability to retrace steps, the ability to not have crucial inventory items get lost (stolen, broken, etc), or even saved game positions.
* Don’t have the game be static. At some point, have the threats be agents that go around the game world doing things as the players are doing things in other places. Like closing and locking doors, creating problems, releasing beasts, or fire or flood waters, or turning off lights/power…