I don't think fossilize or stalagmites accepts -def IOs.
Dark Bard, Zoobait, Debacle
jmsb
Por vezes d� vontade...
chucknorriss
speak american godamnit
That narrows it down then.
@SPTrashcan
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Hell, put it everywhere! 
For my Earth/Thorns, my build is too tight for spare slots, at least for single target attacks. I usually only have 5 slots total. My AoEs usually need all 6 just to get the numbers to decent levels. So, unless I drop something from my Epic, I'll never have spare slots for them. Too bad, seems like it'd be cool.
Lewis
Random AT Generation!
"I remember... the Alamo." -- Pee-wee Herman
"Oh don't worry. I always leave things to the last moment." -- The Doctor
"Telescopes are time machines." -- Carl Sagan
BTW, I hate to threadjack, but I've been trying to decide on an epic for my Earth/Thorns. I'm thinking about going with Ghost Widow, because I usually take Leviathan or Mu on my Dominators. I'm definitely going with Soul Drain and Dark Embrace.
What I'm wondering is, should I go with Dark Obliteration for an extra AOE? Or should I skip it? If I skip it, I could throw the chance for -resist in some of my single target attacks. So, I'm undecided.
Should I go for Dark Obliteration or not?
Lewis
Random AT Generation!
"I remember... the Alamo." -- Pee-wee Herman
"Oh don't worry. I always leave things to the last moment." -- The Doctor
"Telescopes are time machines." -- Carl Sagan
Depends on whether you solo or play in teams. Solo, it's nice to have an extra AoE to follow up with after Thorn Burst. It helps wipe minions out quickly, and since you'll be using your holds on lieuts and bosses this basically stops all incoming damage. It's also nice to have the negative energy damage, since Earth/Thorns is mostly smashing/lethal damage which is highly resisted.
If you play in teams, nevermind your AoEs, concentrate on improving your control and single target attacks. Since just about everything you have is a defense debuff, and Quicksand can't be resisted, you might swap out the 2nd accuracy for a damage on each of your powers, for an overall increase in damage done.
Most dom AOE damage is pathetic compared to corrupters and brutes. The exceptions are Fire/ with /Fire or /Psi. Those two can do good AOE damage, but since the PSW nerf they're both forced to jump in for PBAOE and back out for cone. This reduces the damage from Hot Feet and is just more work than it's worth for the moderate damage increase compared to spamming single target attacks while the PBAOE is recharging. Once they're held, your team is going to steamroll them anyway, so whatever damage you add is just gravy.
Focus on using Domination to control bosses, slotting recharge sets to get it perma, and then just scrap for a bit while the rest of the team steamrolls the helpless targets. Earth has a crazy number of different controls, so you're still an asset to any team without the patron AOE.
My own experience with the achilles heel proc is based on my scrapper, but most of the theory should translate just fine. Actually, the scrapper forum is probably a good place to ask about the proc in general. A lot of the hardcore attack analysis goes on there.
A proc of this nature can be fairly difficult to analyze in detail as it can depend on many things. One point to keep in mind is that, as compared to a damage proc, you are essentially trading damage now for increased damage to come. If, for example, it procs on the last attack you use on a target, it doesn't mean anything. As such AH tends to be more useful against tough targets like bosses and above. It is truly excellent for anyone soloing AVs or pylons.
Keep in mind also that the proc doesn't stack from the same caster last I heard. I expect earthquake provides an exception to this since it functions through a pseudopet, but there is a practical limit to how many times over you want to slot this proc.
For ST powers, a fast recharge attack that appears frequently (and to an extent, early) in your attack chain is probably ideal. You have a good chance of maintaining the proc a reasonable portion of the time.
For AoEs, I would definitely try to use it in an opening power rather than something that is likely to finish enemies.
As such, depending on your specific attack habits, I'd probably slot one into Thorny Darts or Skewer, one into Fling Thorns, and maybe one into EQ if you have slots to burn.
Dark Armor/Stone Melee Tank Guide [I12]
So, I decided to roll an Earth/Thorns Dominator, mainly for conceptual reasons. It wasn't until I took a good look over my power list that I realized - that's a lot of defense debuffs. And of those defense debuffs, many accept defense debuff sets. Which means there are a lot of places I could put the Achilles Heel -Res proc.
Of the following powers which I am taking and which accept defense debuff sets, which would be the best to take an Achilles Heel -Res proc? And am I missing any powers which would greatly benefit from the proc?
Fossilize
Stalagmites
Earthquake
Thorny Darts
Skewer
Fling Thorns
Thorn Burst
Ripper
Thorn Barrage
My current intuition is to put one in Earthquake, Fling Thorns, and Thorn Burst (as AoEs have the best chance of triggering the proc on at least one target), but that leaves me with the ability to place up to two more. I'm inclined toward Thorny Darts and Fossilize as I'd be using those the most frequently especially against hard targets, but I could use some advice.
@SPTrashcan
Avatar by Toxic_Shia
Why MA ratings should be changed from stars to "like" or "dislike"
A better algorithm for ordering MA arcs