unc0ver v3.0.0~b29 is NOW OUT for public testing with full-fledged iOS 12.0 – 12.1.2 support for A8X-A11 devices with Cydia and Substrate.
unc0ver is made by @pwn20wnd & @sbingner
This new jailbreak tool can work on iOS 12 now. Unfortunately iOS 12.1.3 (latest version) is not supported. But you can possibly downgrade or upgrade to an iOS 12 beta release while it is still being authenticated by Apple servers – see the link below for more details. Not all models are supported yet, but will be added soon.
With the unc0ver jailbreak tool, you install the app via .ipa file with Cydia Impactor.
They also have tools for iOS 11 devices. So be sure to check out the Github readme.