So I'm trying to do this thing where the player falls down into the main lobby area of my server and gets an achievement for it. I assume I'd have to place some kind of zone or trigger for the player to fall through and run the command to get the achievement, but is that doable in CC, and if so, how do I do it?

Any help is appreciated.

And what is this "achievement" that they get

8 minutes ago, Goodly said:

And what is this "achievement" that they get

 It's just a simple /award command.

It's not possible without a custom command, since the player would have to be able to give themselves awards for it to work. But to answer your question of how to trigger a command when they fall, you have to add a layer of message blocks. /help mb
If they fall fast, you may need to add more than one layer to make sure they trigger it.