Core is a 3D multiplayer game creator, as well as a platform for distributing and playing those games—sort of like Fortnite Creative, but without a big battle royale game layered on top of it. As of today it's free-to-play in open alpha, so you can try it now.
You can't import your own 3D models or other assets, but you can stick together the provided models to build new things, which can be saved as prefabs and shared with other creators. You can also create and borrow Lua scripted UI elements, NPC behaviors, guns, and so on. While competitive multiplayer games (which currently support 16 players) are the focus, developer Manticore Games tells me you can make cooperative and singleplayer games, too.
During a remote demo last week, I saw firsthand how quickly a simple multiplayer sniping game can be put together: It only took a couple minutes, and required no scripting. The developers also dropped in Portal gun made by another user, and it instantly worked as expected. The terrain editing is voxel-based, so you can sculpt mountains and dig caves into them. Core is built on the Unreal Engine, but its level editor is entirely its own.
The Core client can be launched through a browser call which loads up a specific user-created game, and the games load very quickly. (If you have the Core client you already have all of the assets being used.) That gives Core an interesting capability: Users can place portals in their games which lead to other Core games.
The hope seems to be that some number of user creations will blow up—the way mods like Auto Chess blew up—but I could see Core being fun for Discord groups and other communities who only want to amuse themselves. One idea for how to use it: Have every member of a group design one leg of an obstacle course, link them together with portals, and then race.
As for how Core will be monetized, player avatars are persistent between games, and so premium cosmetics are an option. Manticore Games also says that users will be able to support their favorite creators with 'tips' and subscriptions—the specifics of those systems will come later.
I've tried out a few of the game modes present, and they feel very much like example projects—I'm afraid I don't think Core Royale is going to be the one that takes off. I'm put off by the ugly character models and stiff animations, and it's a little concerning that none of the example guns or UIs in the games I've tried are satisfying to use. But the level editor is fun to mess with, and maybe something exciting can be built around the lackluster elements.