what makes rpm drop rapidly then shut down

If the check engine light is on, then have the vehicle scanned for any trouble codes. Write them down. Any code that shows up would let you know what needs to be looked at. Another reason could be a fuel issue. Fuel pressure test would need to be performed to determine a problem.

engine light is not on

light not on rpm still go down

May be a fuel pressure issue then, fuel pressure test should be done to see what the problem is. Could be a bad fuel pump. or a fuel regulator, or leaking fuel injectors.