Defiance Free to Play

A "Major Arkfall" is a server wide event that randomly happens, a large red circle will appear on the map with 5-6 small red icons inside, players must travel to each one and either protect or destroy a crystal, and then finally move onto a boss, these typically attract large crowds of players so you can expect help, however the more players means tougher enemies.

"Minor Arkfalls" are the little red icons that appear with no huge red circle surrounding it, just watch out for the huge red circles on the map, is possible to solo it but might take a bit longer and require a lot of leveling up.

Depending on the type of Arkfall event it is there are different objectives, there is always a crystal that you typically need to blow up or just survive waves of enemies (not actually defending the crystal), sometimes you can kill Raiders with rocket launchers and then use it to damage the crystal, or wait for a little bug to bite it and open up a weak spot or shooting explosive barrels set up by enemy engineers. You can also just shoot it but obviously critical hits take it down quicker, and there is a time limit.

Servers are apparently region locked so as long as you play during peak hours and not 5 in the morning the "Major Arkfalls" should attract a large crowd and not be too difficult to handle, just try to help as much as you can to make it quicker.

After the minor crystals are cleared out of the "Major Arkfall" area, one final icon will appear which is the boss, the timer resets to 20 minutes so you get plenty of time, there are a few different types of bosses, just shoot the weak spots that glow orange and stick out, if the boss is hiding inside sometimes you need to kill the largest enemy that appears before he comes out again, always focus on the largest enemy that appears.

Edit: As noted in the comments by Onza a recent patch made these events rarer, before there were roughly 3-5 an hour, now its far less.