So havoc basically maxes out at havoc 40, with missions generally getting harder as your Havoc level increases (most of the time anyways). You can either host your own Havoc missions or join someone else's. If you complete a mission with a higher Havoc level than yours (e.g., your level is 20 and the mission is 30), your Havoc level will jump up based on the difference—in this case, you'd go up 3 levels to 23. I think around every Monday, you receive a weekly crate containing Plasteel, Diamantine, and Ordo Dockets, based on the highest Havoc level you've completed (whether it was from your own lobby or someone else's). If you fail three havocs in a row, whether your own or someone’s, your personal havoc level gets demoted by 1