In order to extract the required WhatsApp archive messages.db you need to unlock your Windows Phone, because the standard WhatsApp backup (on SD or OneDrive) is encrypted.

Requirements: any Windows Phone with Windows 10, excluding x50 (like 550,950/XL)

Below the steps to get down to this:

  1. enable Developer mode from your phone's settings (Settings -> For developers)
    Windows phone developer mode on
  2. from your PC download Interop Tools L from this link - unzip then copy to Windows Phone through USB, anywhere you want
  3. on your Windows Phone (not on your Windows PC) tap/click the following files
    1. InteropTools_x86_x64_arm_LegacyRelease.cer (certificate)
    2. all the files into the Dependencies/ARM folder
    3. InteropTools_2.0.95.0.appxbundle (the app itself)
      interop app and certificates
  4. reboot the phone then start Interop Tools and choose "Interop unlock" 
    provider selection interop unlock
  5. enable NDTKSvc and reboot
    NDTKSvc enable
  6. unlock the phone with "Full Filesystem Access" and reboot - on some phone the "New Capability Engine Unlock" needs to be switched too
    Interop Tools - Step 3

Now you can attach the phone to your PC or MAC and copy the messages.db file from the following (alternative) location to the WinWazzapMigrator or Download folder on the Android device.

Phone\Data\USERS\DefApps\APPDATA\LOCAL\Packages\5319275A.WhatsApp_cv1g1gvanyjgm\LocalState

Phone\Data\USERS\DefApps.Windows Phone\APPDATA\Local\Packages\5319275A.WhatsApp_cv1g1gvanyjgm\LocalState

NB: after unlock, if you can't see the phone anymore, please make sure you're using Windows 10 on PC. Older Windows has issues with drivers. Also make sure to have all the latest available Windows 10 mobile updates.

Tutorial: how to upgrade Lumia 530 to Windows 10
Tutorial: how to upgrade most Windows 8 phones to Windows 10

Check if phone is unlocked
messages.db path