Creating a custom Mix task in Elixir is as easy as defining a module in the Mix.Tasks namespace.

defmodule Mix.Tasks.Custom do
  use Mix.Task

  @shortdoc "Runs a custom task"
  def run(kw_args) do
    #do something with those keyword args
    IO.inspect(kw_args)
  end
end

Now, you can run mix custom to execute this task.