ASUS Zenfone 2 Unlock Bootloader

Unofficial Step to Unlock Zenfone 2 Bootloader.

All Credit to Shakalaca, original post XDA

What is a bootloader? At its most basic level, your Android smartphone is like a hard drive, made of up several partitions. One of those partitions holds the Android system files, another holds all the app data you accumulate (which is how you're usually able to update without losing all your stuff), and others to do more behind-the scenes stuff.

Think of the bootloader as a security checkpoint for all those partitions. Because if you're able to swap out what's on those partitions, you're able to break things if you don't know what you're doing. Or, with a little hackery, you're able to run custom ROMs.

Notes:

1. Work for ZE551ML (Z00A) and ZE550ML (Z008)
2. Update firmware first to 2.19.40.12(ZE550ML) and 2.19.40.18 (ZE551ML)
3. Make sure ADB Working properly

How to Unlock Zenfone 2 Bootloader: 

1. Root your Zenfone 2 (Root) 
2. Make sure ADB is Working (Download Here if you don't have it)
3. Connect Zenfone 2 to PC and open cmd at ADB path
4. at Command Prompt Type (press enter after type each line):
adb shell
su
getprop ro.isn > /factory/asuskey
reboot bootloader
5. Phone will restart in recovery, after updating itself phone will restart and ASUS splash screen will be white.
6. Download Black Splash screen (Download)
7. navigate to adb folder opened cmd and type the following command:

adb reboot bootloader (pressed enter)
fastboot devices (pressed enter)
see if device is listed, write following

fastboot flash splashscreen splashscreen.img (pressed enter)
restart your phone and Now black splash screen is back. Congrats your bootloader is now unlocked
8. Done

PS:  you can not re-lock your bootloader again for now

Thanks again to Shakalaca