ED Contagion Hybid League Start Guide from Havoc. These are just some of the main points for easy reference.
Here's the pastebin for Havocs guide. It is excellent with all the information you will need for ED/Contagion. His notes section is particularly helpful and goes into much more detail than this post. Havoc is going CI which is easy to switch from the last tree he uses, but I'm going Dans hybrid. These aren't the only ways to go. You could also go life by taking the scion life wheel. I just prefer Dans hybrid build. It's great.
Tree Progression:
First 22 Points
37 Point Tree
53 Point Tree
69 Point Tree
89 Point Tree
95 Point Tree
Final Tree
Gems Early (3 Links)
Contagion - Faster Casting - Arcane Surge
Essence Drain - Void Manipulation - Controlled Destruction
Blight - Infused Channeling - Void Manipulation
Flame Dash - Arcane Surge - Faster Casting (If you don't have 3 blues use Flame Dash - Faster Casting)
Gems Mid (4 Links)
Contagion - Faster Casting - Intensify - Increased Area of Effect (If only 3 link drop Inc. AoE)
Essence Drain - Void Manipulation - Controlled Destruction - Efficacy
Blight - Infused Channeling - Void Manipulation - Efficacy
Shield Charge - Faster Attacks - Fortify (Shield Charge now works with wands!)
Gems Late (6 Link)
Essence Drain - Void Manipulation - Controlled Destruction - Efficacy - Swift Affliction - Energy Leach/Empower(If you have lvl 3 Empower)
Leveling
Use Freeze pulse until level 4
Use Blight and Contagion to level until 12
Use a combination of Blight, ED, and Contagion for the rest of the game
You can use Bane at lvl 24 instead to level if you prefer that. Bane - Despair - Efficacy - Void Manipulation
Look for a 3 link wands colored BBG or BBB. BBG is the priority though.
Use the chaos +1 recipe. Vendor blue wand + chaos gem to get a wand with +1 chaos. Great news shadow starts with two chaos gems. Don't sell viper strike or poison by accident. You need them for +1 recipe.
Do the Den quest in Act 2 for 2nd Quicksilver
Take the silver flask in Act 5 and spreading rot jewel also in Act 5 (put spreading rot in jewel slot)
Ascendancy
Swift Killer - Ghost Dance - Escape Artist - Weave the Arcane