I'm not too sure about the lore for the older sets. I think phyrexia was destroyed and then new phyrexia was created because karn accidentally brought some of the phyrexians to mirridon with him and they turned mirridon into new phyrexia. Please correct me if that isnt right.
I've read the 3 Mirrodin books some time ago, at least for this block I can help a little with the lore:
Karn build Mirrodin and by accident left some Glisterning Oil there. It has first reproduced silent and then startet corrupting Memnarch (Karn left him to watch over Mirrodin) and the rest of the World. This also caused metal turning into flesh. Memnarch got defeated by Glissa after a hard fight, but afaik this hasn't removed the corruption.
Since this is the end of the Mirrodin books, I can't tell how it exactly turned into new Phyrexia, or what happened to it.
I forgot I posted in this thread, so i haven't been keeping up. Oops.
So is this why Memnarch is able to turn anything into an artifact? Because he was corrupted by the glistening oil. And the glistening oil is the same thing that fuels, or created, not sure the terminology here, SKithiryx, the Blight Dragon?
Okay, I'll try to remember some details:
Mirrodin was, after
Karn created it, a full metal world. Qucksilver seas, Copper Mountains, etc. And everything living on it was also mechanic. After some time,
memnarch used soultraps(?) to capture living beings from other planes, and brought them to mirrodin, in order to bring some life on this dead world. But with the ages, and the corruption spreading, mirrodin changed. First, the living started slowly transforming into metal. As you can see on the artwork for
Glissa Sunseeker, her arms and legs are covered by metal, other parts of her body are
flesh. This is no armor or something like that, thats her skin. A result of the corruption spreading on mirrodin.
But the corruption also worked in the opposite direction. Later on, close to the birth of the fift sun, something changed, and pure metal beings, like
Memnarch or
Bosh, Iron Golem started turning into
flesh.
about
SKithiryx, the Blight Dragon: It might be a pure Metal dragon that turned into
flesh, or maybe a dragon (or more likely descendant of a dragon),
Memnarch brought to Mirrodin.
If you want to read about all the details, there are 3 Books that cover the Mirrodin story. They are no masterpieces, but still worth reading, especially if you want to get a little deeper into the magic lore.