I heard the music of dragons den basement 1 floor and really liked it. It got me thinking about a really scary sound.

 

The sound of a heartbeat...

 

Imagine this heartbeating slowly and going faster when you reach your destination, npc, boss npc or in this instance, kyogre or groudon.

 

Now this heartbeat concept is very simple but it could be very effective too i think.

Infact, if you want to add to the heartbeat. The screen could fade-flash red when it goes faster even.