Any game engine good for being embedded in Elixir?

You could do something simple like raylib. There are some existing projects integrating with it. There are also idiomatic Zig bindings!

Charms looks like a very cool project, I’ll be keeping a closer eye on it.

For a game engine, I think Sokol matches what you are looking for. It’s not exactly a game engine rather than a collection of STB-style (header only) libraries. They are (1) cross platform, and (2) fairly easy to understand and port.
It’s not a complete game engine in the sense that it has a physics engine, or PBR rendering, etc…

Another one that I had explored recently is Gunslinger It has some basic physics utilities as well.

The biggest problem with embedding a game engine in the BEAM is that preexisting ones, even embeddable ones, generally have preexisting assumptions about the runtime of your application that don’t mesh too well with BEAM’s. Especially since platforms like macOS (last time I checked) require the main thread to be the only windowing thread. It might make the most sense to have a port that handles windowing and draw calls and convince that with NIFs that do related computations in the BEAM.

So this doesn’t work

defmodule WinRaylib do
  use Charms

  defbind init_window_wrapper() :: i32()

  defm init_window(env) :: i32() do
    init_window_wrapper()
  end

  def dynamic_libraries() do
    so = Path.join(System.tmp_dir!(), "raylib_test.so")

    {_, 0} =
      System.cmd("cc", [
        "-shared",
        "-fPIC",
        "-o",
        so,
        "test/support/raylib_test.c",
        "-I/opt/homebrew/include",
        "-L/opt/homebrew/lib",
        "-lraylib"
      ])

    [so]
  end
end

#include "raylib.h"

int _mlir_ciface_Elixir$RaylibTest$ScreenWidthTest$init_window_wrapper() {
    int width, height;
    if (GetScreenWidth() > 0 && GetScreenHeight() > 0) {
        width = GetScreenWidth();
        height = GetScreenHeight();
    } else {
        width = 800;
        height = 600;
    }
    const char *title = "Test Window";
    TraceLog(LOG_INFO, "Initializing window: %dx%d, title: %s", width, height, title);
    InitWindow(width, height, title);
    TraceLog(LOG_INFO, "Window initialized - IsWindowReady: %d", IsWindowReady());

    if (IsWindowReady()) {
        TraceLog(LOG_INFO, "Entering window loop");
        // Keep window open until ESC pressed
        while (!WindowShouldClose()) {
            TraceLog(LOG_DEBUG, "Window loop iteration");
            BeginDrawing();
            ClearBackground(RAYWHITE);
            DrawText("Test Window Open", 20, 20, 20, DARKGRAY);
            EndDrawing();
        }
    }

    return IsWindowReady();
}

void close_window(void) {
    CloseWindow();
}

Running ExUnit with seed: 20931, max_cases: 16

.........................................INFO: Initializing window: 800x600, title: Test Window
INFO: Initializing raylib 5.5
INFO: Platform backend: DESKTOP (GLFW)
INFO: Supported raylib modules:
INFO:     > rcore:..... loaded (mandatory)
INFO:     > rlgl:...... loaded (mandatory)
INFO:     > rshapes:... loaded (optional)
INFO:     > rtextures:. loaded (optional)
INFO:     > rtext:..... loaded (optional)
INFO:     > rmodels:... loaded (optional)
INFO:     > raudio:.... loaded (optional)