How does it work exactly? Ive had a bit of a search but everything I find is either a video or a hideously optimised website that crashes my mobile browser.

I found a green key of some kind, but I've no idea how. Are they just random patrol drops like Runes or Skyburner keys?

There was also mention of different "purity" levels I think? Are they like different levels of runes in CoO?

I realise the expansion's only been out a day, so I dont expect a complete rundown, just bored at work and unable to find any good info.

The short answer is "Yes."

Uncommon, Rare, and Legendary random drops. They can even drop during Archon's Forge. You can only carry one at a time, but can be stored in your vault, so you can bypass that limitation. I've only run a single Uncommon one, but that was because I went in Solo. I didn't get any exciting drops from the "Bosses." But it was a nice little frenetic thing. Probably better with a group.

How does it work exactly? Ive had a bit of a search but everything I find is either a video or a hideously optimised website that crashes my mobile browser.

I found a green key of some kind, but I've no idea how. Are they just random patrol drops like Runes or Skyburner keys?

There was also mention of different "purity" levels I think? Are they like different levels of runes in CoO?

I realise the expansion's only been out a day, so I dont expect a complete rundown, just bored at work and unable to find any good info.

I mean, I get the basics. You find keys as random drops, and there are different levels of keys that determine the difficulty of the enemies you face. I'm not sure if you can only hold 1 key, or up to 1 key of each type... but either way, that seems like an odd decision to me (my first thought was that it must be a bug). I was in the Forge with a group of 5, and after we'd all used our 1 key each, we couldn't continue because none of us had received fresh keys as drops over the course of playing the activity.

So I get the basic setup. What I don't understand is the rest of how it works: there appear to be different "styles" of encounters that can be summoned, but are those just random? Are they specifically tied to the difficulty level of the Key you use? What the heck are these "boons" that one of the bounties requires, but the activity itself does not explain? What's with the door that closes and locks out players who don't enter the arena the moment the activity starts, and then requires them to spend a key of their own to get back in?

I mentioned in one of my posts yesterday how much this expansion reminds me of House of Wolves in several ways (I suspect RoI was at least partially developed by the HoW team within Bungie), and the Archon's Forge is a great example. It feels slapped together and poorly thought out, much like the original Prison of Elders. Except at the very least, PoE had unique bosses to spice things up a bit. The Archon's Forge bosses are just regular Major enemies that you can often kill with a single super.

Is there a dynamic going on that I'm totally missing?

How does it work exactly? Ive had a bit of a search but everything I find is either a video or a hideously optimised website that crashes my mobile browser.

I found a green key of some kind, but I've no idea how. Are they just random patrol drops like Runes or Skyburner keys?

Yes.

There was also mention of different "purity" levels I think? Are they like different levels of runes in CoO?

Yes.

I realise the expansion's only been out a day, so I dont expect a complete rundown, just bored at work and unable to find any good info.

We did it a few times last night; I got one sublime engram from the first one, but just greens and blues from the others.

At some point a message will appear that says "You will be tested..." or something like that. I've no idea what the damn test is, but after a little more playing the message "Impressive!" appears and then one of the domes will generate 3 axes for you to use. I'm assuming it's a test to see how much dps you can put on the bad guys in X amount of time or whatever.

There are also a few different levels of difficulty for the forge. We did one where the target was shanks and two where it was dregs, all from the green keys. Apparently there are also blue and yellow keys for "harder" bosses.

All in all it was very easy and very quick. Seems like a good spot to farm engrams.