The original Adhell app was marketized as a system wide adblocking app without VPN, but it can do more than just blocking ads. It can also disable system apps and disable app's permissions.
Adhell is working by utilizing Samsung's Knox SDK. Therefore, it only works on Samsung devices.
Within this SDK, there is a firewall function which can be used to block domains or define firewall rule to block a particular port for a specific app or restrict the internet access.
Adhell is merely a front-end for Samsung Knox SDK which initilizes the Knox with what you define (blocked domains, firewall rules, whitelists, disabled apps, etc). Therefore it doesn't run in the background.

Adhell was developed by Samsung's developer. After he was forced to remove the code from internet by Samsung, FiendFyre was stepped up by providing the Adhell2. But after a while, it is also discontinued.
Adhell3 is an extension of previous discontinued Adhell2 app with more additional features.

• Mobile internet blocker
  Disable internet access completely when you are on mobile for specific apps. This can be useful to avoid watching video accidentally by using mobile data.

• Custom deny firewall rule
  This can be used for example to define a custom firewall rule to block ads for Chrome app on port 53 for all ip addresses:
  com.android.chrome|*|53

• Whitelist URL for a specific app
  When you have a domain that you want to block system wide, but you need this domain on a particular app. Otherwise, the app won't work.
  Instead whitelist-ing this app, you can just whitelist that domain for this app.
  Example: Block the domain graph.facebook.com system wide, but allows it for Facebook Messenger so that it can be used for login:
  com.facebook.orca|graph.facebook.com

• Support local host source
  The host file can be located on internal or external storage.
  An example to use host.txt file which is located at internal storage:
  file:///mnt/sdcard/hosts.txt

• Show the content of host source
  Show the list of domains of individual host source or the list of all blocked domains from all host sources.
  This can be useful to check whether particular URL is in the list.
  The list contains of unique domains.

• Retain data across installations
  Adhell is usually generated with a unique package name. For the users that don't build Adhell with Android Studio, they cannot use the same package name. This means that they need to reinstall Adhell every times there is a new version and on every installation, they need to input the same data again and again.
  With this feature, there is no need to export or import functionality again. The data will be kept on internal storage and by next installation, the same data will be used.

• Clone the project with git clone [email protected]:fusionjack/Adhell3.git adhell3
• Download the source code as a zip file: https://github.com/fusionjack/Adhell3/archive/master.zip
• Set applicationId with a valid package name in app\build.gradle, e.g.: com.dhf.erz58384

• Download and install latest Android Studio from https://developer.android.com/studio/index.html
• Open the downloaded project in Android Studio
• Install missing SDK and build-tools

• Update the source code with git pull --rebase or re-download the source code as a zip file
• Run these following commands in a console:
  

cd adhell3
gradlew clean assembleDebug

• The generated apk is located in app\build\outputs\apk\debug

You need a license key in order to use Adhell. You can get this license from Samsung for free but it needs to be renewed every 3 months. Don't forget to revoke the old license key before creating a new one.

• Enroll as a developer with this link: https://seap.samsung.com/enrollment
• Generate a license key with this link: https://seap.samsung.com/license-keys/create#section-knox-standard-sdk
• Choose Enterprise License Key
• Give an alias name, e.g. test
• Click on Generate License Key

With Knox Standard SDK: https://seap.samsung.com/sdk/knox-standard-android

Yes

No

Blocks ads system wide.

Adhell is free and open source.

Disconnect Pro and Adhell are using the same underlying Knox Standard SDK. If Disconnect Pro is running Samsung Firewall Adhell doesn't have rights to change Firewall settings and vice versa. So they can't work together at the same time.

No

No

You may see some ads.

Adguard (without root) will set up a local vpn to route adds to nowhere basically. I think with root it uses a proxy. Either way, it's different than how Adhell does it, so they should work side by side just fine.