A deep dive into Indigon: How to do (acceptable) damage on the cheap

TLDR:
Indigon for the most part does not give more damage but enables cheaper builds to perform acceptably.

Good with Indigon: mana regen, cast speed, cost increase penalties
Bad with Indigon: spell level, % increased damage, cost reductions

Build (not optimised) : https://maxroll.gg/poe2/pob/kn9la0jj
Mapping and map boss: https://www.youtube.com/watch?v=GrqTyycOzXQ
T4 Olroth: https://www.youtube.com/watch?v=H59j2UYzIfs

The most expensive item used is a 12 div Maligaro's. Along with that a 4 div tri-res belt, 3 div boots, and 1 div Against the Darkness are needed just to cover resists. Using rare gloves would make everything 30ex or less. Similarly the 4 div Bonestorm Prism isn't really needed, you can put on a Zenith support gem instead. Without Maligaro's more crit nodes would be needed on the tree, and I estimate around 30-50% DPS loss, but should still be able to do the same content, just slower.

The following is a long analysis of Indigon mixed with build ideas. I will preface by saying I am not a PoE1 player. I have zero minute in PoE1. A lot has already been written about Indigon in PoE1, where I believe spell mana leech exists, making Indigon's mechanic inherently broken. I will only talk about the PoE2 version of Indigon, which I think is the most interesting new unique in 0.2.0.

What Indigon does:

35-50% increased Cost of Skills for each 200 total Mana Spent Recently
35-50% increased Spell damage for each 200 total Mana you have Spent Recently
Mana Recovery other than Regeneration cannot Recover Mana

What Indigon really does:

Despite the increased spell damage in its effect, with a few exceptions Indigon does not improve DPS. It is in fact a large DPS loss for almost all spells. Instead Indigon allows cheap builds to be viable in the end game. Building with Indigon for the most part is trying to close the DPS gap to a normal high end build with low end gear. To understand this let's start by examining damage in PoE2.

Spell level tax:

Fundamentally a caster is a machine that converts resources into damage via spells. A player's theoretical maximum sustained DPS is given by the formula

(resource gained per sec)*(damage/resource ratio of spell used)

and the maximum burst DPS by

(mana pool + resource gained per sec)*(damage/resource ratio of spell used)

The most common resources are life and mana, which can be gained via leech/regen/potion/on kill effects/recoup etc. In the 0.1.0, only the burst damage matters since Melting Maelstrom is essentially infinite resource per second, and bosses die in a second anyway. We can also see why Bloodmage becomes the best caster ascendancy this patch. Spell leech is a fundamentally broken mechanic.

For Indigon users, the game is simple, our resource is mana, and it can only be gained via regen. The damage/mana ratio of a spell, or its efficiency, can be increased by various multipliers. Indigon demands constantly casting and using all our mana, so the sustained DPS formula becomes more important:

(mana regen per sec)*(base efficiency of spell)*(increased damage)*(more damage multipliers)

To do more DPS, we want more mana regen, or improved efficiency with multipliers, preferably both.

If you go through the list of PoE2 spells, you will find that the base efficiency of most spells stays pretty much constant throughout all levels. High level spells may have some extra benefits like larger AoE, but the main damage numbers mostly stay at a similar ratio to cost. In fact efficiency tends to decrease at the very high levels. That is how the game is balanced on a fundamental level. Those complaining about the high mana cost of high level spells need to understand that building mana recovery is as necessary as building defense.

So why is spell level the most coveted stat and costs an arm and a leg? In practice there is another resource which creates a bottleneck in achieving the theoretical maximum DPS: time. We need to be able to actually spend all that mana within a second. This is where cast speed and spell level come in. Another way to spend more mana is by triggering spells. Caste speed, skill level, and energy generation should be viewed in the same category. They all allow us to spend more mana per second. All three are interchangeable so long as we can spend all the mana regen. What they don't do is increase our theoretical maximum DPS, they just allow us to reach it in practice. There are very few sources of more speed, and it is hard to scale cast speed beyond +300%. Energy generation has been nerfed to the stone age and only works well if DPS is already high enough. That leaves skill level, which is always multiplicative with itself, as the main way to funnel mana.

The main benefit of Indigon is the removal this bottleneck, and thus the gearing dependence on spell level. Spells will cost more to do more damage. A low level spell can scale up in cost to use all the mana available. We no longer need to have spell levels on wand/focus/amulet. Gears without spell level cost in exalts instead of divines. We are essentially free from the spell level tax.

Let's look at an example. Say we have an average 50/50 Indigon, and a mana regen of 1000. This means as long as we spend all our mana, our spells will have 1000% increased cost and do 1000% increased damage. Suppose we have 140% increased cast speed to cast Frostbolt 3 times a second. Without Indigon, we will need level 33 Frostbolt to spend 1000 mana in a second, that's Prism territory. With Indigon, we need level 14, which you can find on the floor in the campaign. You can see how much we stand to save using Indigon.

DPS loss:

However this comes with a huge downside. An average Indigon doesn't actually improve the efficiency of our spells. At the same time it dumps a huge number into the increased damage pool, effectively removing increased damage as a scaling factor.

Continuing with the last example, say we have 500% increased spell/cold damage between our gear and tree. Normally this will result in a 6-fold increase in efficiency and DPS of Frostbolt. With Indigon, since we already have 1000% increased damage, a further 500% will only increase our damage by a factor of 16/11=1.45. Effectively our 500% increased damage investment is only giving us 45% more DPS, an over 10-fold loss, ouch.

Things get a little better if we use a better rolled Indigon. Say we have a perfect 35/50 Indigon, our spells still do 11 times the damage but only cost 8 times more. This gives a 1.375x multiplier to efficiency. In practice this means we use level 16 Frostbolt to still consume 1000 mana a second. Now having 500% increased damage will result in 1.375*1.45=2 times the damage. This is still only a third of the damage of a build without Indigon.

There is another less obvious but equally devastating loss of DPS. The large increase in spell cost from Indigon also renders cost reduction effects useless since they are additive. Cost reductions are additive and they normally scale hyperbolically. Mental Toughness, the Conservative Casting cluster and Deep Trance (this is very much worth it if your path around that area, even with two dead attack speed nodes) together will result in a DPS multiplier of 1/(1-0.15-0.16-0.1) = 1.695. This scaling factor is also gone if we use Indigon. Note that multiplicative more/less cost effects such as Inspiration still work the same regardless of Indigon.

In conclusion, when using Indigon, we should not invest in increased damage or cost reduction on gear or tree, as their return will be abysmal. We therefore completely lose two DPS scaling vectors, including the best/easiest one. We are basically starting an order of magnitude down in DPS. I think this is why Indigon is not widely adopted by the player base. If you put it on in a normal build, it is going to feel bad, real bad.

Sidetrack - Support Gems:

Let's take a closer look at support gems in general. Since efficiency is what really matters in determining maximum DPS, a lot of support gems are worse than they initially look.

Take Brutality, it gives 25% more physical damage. However it also has a cost multiplier of 120%. This means it only improves efficiency by 5/120= 4.2%. Is that really worth the trouble of giving up the damage as extra multiplier? In general it's very hard to justify a damage support that has a cost multiplier. I used to put Magnified Effect everywhere, but it has a hefty 130% cost multiplier. Is 30% more area worth the 30% hit to your max DPS? Could you instead cast 30% more spells to cover that area? Of course Magnified Effect is still very much worth it in many cases, however it is a much tougher decision than first thought, especially with Indigon where you WILL use all your mana.

Another interesting support is Considered Casting. It has 15% less cast speed which might be viewed as the major drawback. However, cast speed actually doesn't change the maximum DPS since it does not change the efficiency. Sure you now cast less times a second, but you can make up for it by putting in a higher level gem. You still spend the same amount of mana to do the same DPS. Less cast speed just makes the build less snappy and generally feeling worse. The actual hit to DPS is the 115% cost multiplier. The effective DPS multiplier of Considered Casting is 135/115 = 1.174, not bad but actually worse than something like Deliberation, which is a flat 20% more with a similar clunkiness downside.

Building with Indigon:

We will assume that compared to a well geared caster build without Indigon, we are doing 1/10th of the DPS due to not having an increased damage multiplier or a cost reduction multiplier. We are always playing catch up, and we need to optimise our build to close the gap. The following is a list of general ideas and specific interactions that work well with Indigon.

1. Mana regen. First and foremost we need mana regen, both base regen from total mana (life for lich) and increased. All our gear should have T1 mana regen where possible. Fortunately regen is not a meta mod, T1 mana regen on gear costs practically nothing. It is pretty easy to get over 1000 mana/s under arcane surge (a must have). We will have a mana advantage over non-Indigon builds. I would estimate about 30%-50% more mana. This translates to 30%-50% more DPS. However this advantage will diminish at the top end, since it is possible to get T1 mana regen with +5 spell levels and double T1 increase damage on the same wand, it would just cost hundreds of divs.
2. Waveshaper. Since we stack mana regen, the 1 ex Waveshaper unique chest is a very good defensive option. It pretty much solves sustain on its own. We get an insane amount of ES recharge rate, full ES recovery in a second or so. All we need is one cluster of faster ES start and a couple of jewel affixes to get ES delay down to around 1.5s. Between that and Convalescence we can grind boss fights as long as we don't get one-shotted, a couple of dodge rolls after a large hit and we are back to full ES.
3. Detonate Dead. There are some spells that don't scale well with level, primarily Detonate Dead. I am certain Indigon is BIS for a dedicated Sacrifice/DD build. I would gladly pay 10 times the mana to give DD 1000% increased damage. I tried a DD build with the 10 spirit Terracotta Soldier. It actually steamrolls T15 maps. Unfortunately it is fighting for the same expensive minion level+spirit gear with all the minion builds, which kind of goes against our goal of making cheap builds. I will probably revisit this later when I have more currency to throw away.
4. Damage as extra. Can replace increase damage affixes on wands with these, since Indigon gives generic spell damage, all elements benefit the same. Lich also has unholy might. Mana Tempest fits the build very well with a lot of extra lightning damage. One problem with Indigon is at the start of a battle it takes time to ramp up mana usage. Mana Tempest eats mana like nothing and is the perfect opener.
5. Free spells. If a spell costs zero, then the increased spell cost doesn't matter and Indigon is all upside. We have the usual suspects Choir of the Storm and Corpsewade. And then there are auto attacks from wands. These have serious potential since they have infinite efficiency. We just need to figure out a good way to spend mana while spamming zero cost spells to ramp up Indigon. Bone Blast appeals to me the most with a high base crit. Cast on crit Bone Blast+Choir of the Storm could be quite a fun build.
6. To take it even further, we could utilize the other broken mechanic: spell leech. Using the Covenant robe we get 5% spell leech. Since Bone Blast costs zero it is just constant free life leech. To make use of all the life gain we can put on another cast on crit with lifetap and essentially go infinite if we can improve the efficiency of the CoC spells to leech over 150% of its base cost. I have not personally tested this but the idea seems sound as it would fully utilise both life and mana to maximise DPS output. A lot of in game testing would be needed since I do not know the leech resistance values to do a proper calculation to see how feasible this is.
7. Flat added spell damage. This is basically free spell, and it works best with low level spells to give a higher percentage damage increase. The only source I know of is Flame Wall. Flame Wall is still extremely powerful even after a 50% nerf. Projectiles are just generically powerful with or without Indigon since they offer additional more damage multipliers. Builds using Sire of Shards fit well with Indigon as they naturally lose 7 spell levels by using a staff.
8. Increased spell costs. For the same reason that Indigon heavily reduces the benefit of increased damage, it also significantly reduces the penalty of any increased costs since they will be overshadowed by the huge increased cost from Indigon. These passives are really good with practically no downside: Barbaric Strength, Raw Mana, Overzealous. Get them if you can.

This is getting extremely long. I will save the analysis of Bonestorm and its interaction with Indigon for another post. Thanks very much for reading if you get this far!