Engine control mick module. If the engine control relaly's output is not there, the ECM is not powered up, it is as simple as that.
The ECM fires the plugs and fuel injectors. WIthout them operating, the engine cant run.

Additionally, the ECM enables the starter (and disables the starter when the engine gets to 400 RPM), without the ECM powered up, the starter is not going to engage.