Have a look at bazaar. Look for: Elaborate Defiant Chain gear. (Recommended lvl 70).
On AB they are now around 50p per slot.
Purity was introduced in the Era "The Buried Sea". You gained a new slot, where you can stic one of the new powersources. The powersources drains there power and need to be replaced.
The higher the value of the purity, the faster the power will drain and the more stats you gain out of the powersource. Those powersources come in different flavors.
One might increase some of all, another is big in gaining mana ..., a nice starting point for those is:
http://www.eqwiki.net/wiki/Combine_Power_SourcesThe impact is not very big, nothing you need to spend alot of time to, trying to get back in the swing. Have a look in bazaar. Nowadays you can get some of the playermade powersources there.
From the playermade powersources the Earth Orb may be a good starter.
Corruption is just a new resist, like poison and disease .. nothing you should focus on. The resist mode of this on gear is low and will stay low for a long time. For the Goup-Game it has no big relevance.
When you are going to raid, you will have a cure for it
RK.: From the Expansion TSS Spells are now coming in 3 flavors. RKI are meant to be archieved quiet easily, you are up and going. RK.II can be archieved in the group game, some do drop, others will need faction work. Simply same spells, but some more powerful. The RKIII-Spells can (mostly) only earned via raids. This is a simplification but generally describes the idea. Where and how to get your spells should be able to be found on this board. If not just have a look at allakhazam:
http://everquest.allakhazam.com/How to restart: I think best would be to find a Guild and Friends who will than help alot to get you up to date.
If you want to try a little on your own here some acutal hotzones.
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55 Blackfeather roost
60 Plane of Storms
65 Sunderock Springs
70 Riftseeker's Sanctum
Sunderock Springs is a nice place, and you should be able to find targets for Root-Rotting. As it is a hotzone you maybe can also find a Group there.