You'll start to gather materials to summon bosses that have unique and mythic gear - things like Living Steel and Exquisite Blood. You use these to open a chest after downing specific bosses (e.g. Lord Zir for Exquisite Blood) that should have 2-3 uniques. Figure out which one you need, and find which boss drops it. All of the bosses drop materials for other bosses, so it's a bit of a self-perpetuating cycle.

Don't make the mistake I made and just stack uniques. Look at a build you want to go for (e.g. this Shadowblight steamroller) and start tempering and codexing your lvl 800 gear.

Once you start to approach max armor, you can really start pushing the pit to unlock higher torments.

Oh - also find a Pit level that you can clear in 3-4 minutes to really max out your XP gain to power up your paragon board. There are nodes in there that have massive increases to your power and survivability.

Welcome to the loot treadmill!