superuser apk is the most needed app on rooted Android devices. Superuser download cannot be done in the normal way which you install any other app on your device. You must have rooted your device properly for download superuser. Superuser is available in both format as Binary and App. But app is frequently updating than binary. You can download Superuser on your rooted device below.
There are several ways to download superuser on your device. Installing a custom ROM will get it also. Otherwise you can visit this website using a rooted mobile device and download superuser apk or Download superuser latest version on your PC and transfer it to Android device via Data cable. Alternatively you can install superuser directly from Google official appstore.
You can't uninstall / remove superuser from your device if it came along with custom ROM. In other hand there's no point of having rooted device if you remove superuser.
@Chainfire, @ChainsDD, @ArcMan09