So, a few months ago in March, Crytek announced that CRYENGINE V would be licensed under a “pay what you want” business model, which extends down to free — nothing up front and no royalties. It supports DirectX 12 and a Mono-based framework, which they're calling CE# Framework, that allows gameplay code to be programmed in C#. Since it's done in Mono, it looks like it can be used in all supported platforms, but I could be wrong. While C++ is typically more desirable for AAA-style games, other engines, especially Unity, have attracted a lot of attention with their C# parsers.
The engine doesn't appear to support Vulkan, though, at least not yet.
Hold the phone…
Today's news? The source code is now on GitHub, and not even as a private repository. It's just… there. CRYENGINE V is licensed under a typical EULA, of course, so they impose a few restrictions on how it can be used. Content must not be sexual explicit, vulgar, or “in a reasonable person's view, objectionable.” I expect that this will not be enforced too strictly in terms of violence and cursing, but it differs from, say, Unreal Engine 4, which officially permits Adult content (although they'll occasionally ask to have their trademarks removed, so their logos do not appear to be endorsements).
Crytek also prevents their engine from being used in simulation, science, and architecture. I assume those are intended to be pushed into a separate licensing structure. It would seem silly for them to just outright ban those applications.