Unreal Engine 4 is a game engine made by Epic Games. The Unreal Engine has been around for over a decade and has been used to make a ton of games. The latest release of the engine (4) has a new business model too: it’s free!
You can download and make games right now and release them commercial. All Epic asks is that you pay them a royalty of your sales.
To get into Unreal 4 here’s some helpful resources.
If you’re used to using Unity you’ll want to check out this handy guide on using Unreal 4 for Unity developers.
Good sources to learn Unreal right from Epic:
Within Unreal there is a visual scripting system called Blueprint. Here’s a good tutorial on getting started with it.