Why is my iPhone 8 getting super hot when I’m playing just one certain game combat master

Since it’s just that one app, apparently the app is requiring a lot of power for computation, graphics, communications, or all three.

As the app’s power demands seem excessive to you …

… suggest you contact the app’s developer to determine if it can possibly be run in a “less energy intensive” manner or mode.

How to contact an app developer - Apple Support

In general, the harder that an app works a processor, the more waste heat the processor will generate. (Whether that's due to the processor doing real work, or whether that's due to bad coding.)

Maybe this game is working your iPhone harder than your other apps do?