How to make ambient music slowly more intensive

I making horror game and i want to make ambient music more and more intensive while player going trough the map.

Make your composer create multiple stems, with some mixing order, where you bring in each stem when you need to step up the intensity.
Start them at the same time, and make sure the loops are exactly the same length, so they will stay in sync across the level.
Control intensity by controlling the mix of the stems. Your composer may be able to give some suggestions about specifics based on whatever the music is.