So I had an idea: I wanted to try and see, if you had the absolute best gear, how much ES you could potentially stack and how well that would convert to damage via different Energy Shield scaling damage abilities, of which there are two that come to mind: Righteous Fire and the new skill Energy Blade. My initial plan was to scale very high ES for the new RF ES scaling for mapping, and then flex into using Energy Blade with Ice Crash into Ignite for bossing. Unfortunately, many things went wrong here, and the build is basically dead on arrival.

That being said, this was an experiment, not a serious 'league starter' attempt, so plenty of damage passives and gear mods were sacrificed for ES, so the low damage is somewhat to be expected in return for the absurdly large amounts of Energy Shield gained in return.

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First, the cool idea: using Energy Blade as a bossing tool. At full build, we will have over 40,000 energy shield! Had Energy Blade snapshotted or scaled better, we could have a weapon with upwards of 70,000 added lightning damage. Of course, since we went all-in on energy shield, mana, and int stacking, we don't have the passives or gear to scale much off of that. But we do have some fire damage, some DoT multiplier, and some elemental damage increases, so we can use that to buff ignites. We swap out a ring for Stormfire, we use Ice Crash since it gives us the biggest multiplier on our base damage, and we're looking at upwards of 18 million damage per ignite! Pretty good bossing damage for an RF build!

And then the gem info released, and it turns out we only get 1 million damage, which is like half of what RF does. So the Energy Blade idea is dead, unfortunately, and since we have almost no other skills that scale off of Energy Shield (Forbidden Rite being the only other real contender, but that scales off Chaos, not Fire, so our build doesn't do anything for that), our dreams of having RF for mapping and another skill for bossing, both scaling off of our massive Energy Shield pool, is dead for now. Maybe that will change if they do a mid league buff for Energy Blade.

I'll drop the dream PoBs here (these do NOT work with the current Energy Blade gem, so do NOT use these for a build; they're just for reference for the dream that could have been): a mirror-tier gear setup (https://pastebin.com/VPv5SAua) and a still very expensive but somewhat more reasonable setup (https://pastebin.com/8z2Kp2Hc)

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Second, Righteous Fire: there are other builds with more damage, but I think it'll be hard to find builds with significantly more energy shield. I went with a full int stack, mana stack, and then build energy shield off of a lot of '% Max Mana as Maximum Energy Shield' modifiers, as well as the usual int stack %increased energy shield modifiers. With the 15-20% ES gear buff that will be added, this build will exceed 40,000 ES with mirror-tier theoretically perfect gear (https://pastebin.com/xutbMMrF) and only around 18k for simply very strong gear (https://pastebin.com/jcXgrTM4) (note that I ignored resistances and flasks for the builds, since the gear and flasks are flexible in that regard and don't really tie into the energy shield stacking much).

Unfortunately, as one can see from investigating the numbers, while the mapping damage isn't bad (nearly a million for the strong gear variant, around 3 or 4 million for the perfect gear variant - you'll need to adjust the damage by about another 50% from what's shown to account for ES scaling buffs that aren't on PoB yet), the Sirus damage is pretty abysmal.

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Overall, my conclusion is that a pure ES stacking build does not have the potential to be very strong damage-wise, even with the new and improved ES scaling on Righteous Fire and the new Energy Blade skill. They would need to buff pure ES scaling, or else give us more passive points, as the gear modifiers are almost all taken by intelligence/mana/ES stacking modifiers. Without those, this massive ES pool isn't possible to achieve; but with those, there's no room for further damage modifiers. Since Path of Exile damage is very much about stacking different increased/more/multiplier/resistance damage sources to attain large damage values, dedicating so much of the gear and passive tree to ES stacking heavily lowers the potential damage the build could do.

Of course, if anyone has ideas to improve damage, feel free to theorize below. But I suspect most of that will involve lowering the ES by a lot, which kind of goes against the experiment of this build. In the end, I would not recommend this build besides being a 'meme' build that derives satisfaction from having absurdly high Energy Shield. I probably will go for a more damage-oriented version of RF which doesn't stack to such high ES levels and will only turn to this once I have the currency for such a meme build, but it's fun to think about how high we can possibly get and how much scaling damage we can do off of that.