Mastery Mode, what is it?

[Xalaron]

What is the mastery mode option available at the end of the mode selection? Why would I do it, are there good rewards?

The rewards tier thing at the top with the specific challenges immediately turns me off of w/e this is. I figured I should give it a chance though and actually learn about it in depth as it may be vital for endgame for all I know. Just returned to the game and theres just so many different modes the game is feeling watered down. Do this mode for this, that mode for those, try blah mode for blah.

[prpl_mage]

Chaos and Onslaught are basically the same thing. Just different formats.
Incursions are special stages that just unlocks some legendary weapons at the Wayfarer, apparently the weapon skill on these weapons no longer fill up a shard slot.

Mastery is basically all maps on the different tiers you've unlocked with chaos or onslaught. As you progress there, more tiers will be available. You complete the stage and earn either 1, 2, 3 or 5 stars depending on how well you stick to the limitations. Just completing each map will get you a star, and completing all stars in a single run earns you the 5th.
Each 10 stars you gather you get a reward. For the first 100 I think it is a Hero slot, a pet bundle and a shard bundle and then at 50 you get Mass Destruction - a shard that is twice as usefull as the Destruction Shard.

When you have completed the first 100 stars it starts over with new rewards, but each time you reach 50 I'm pretty sure you get a new Shard at least. And they're worth it.

[Xalaron]

The better shards seem interesting, but the real question is do they persist through ascension when you reset? If you have to go through the trouble of earning them each and every ascension it seems like a waste of time until you dont feel like reseting anymore.

[prpl_mage]

I'm not sure since I haven't done it myself yet but I'm pretty sure you get to keep both Shards and Hyper Shards when you reset. Only the gear and relics that are capped? I don't see how people would go through with the reset otherwise.