Impending Doom + Cursed Ground

Probably more like a too powerfull unintended synergy than a bug.

When you combine impending doom with cursed ground, you can trigger impending doom on every cast after the first without any investment into reduced curse duration or curse activation time.
Meaning only your mana and castspeed limit how often you can proc impending doom.
On youtube you can find someone doing t10 with a lvl 3 enfeeble curse for minimum base manacost.

I guess this is unintended because with this interaction (easily 4x impending doom triggers/per second with supports like spell echo) reduced curse duration on the tree is a completely unneccessary stat. Even considering poison builds that want one big hit these two are better.

I tried reduced curse time and activation time, then this combination and compared them. The damage is at least 10x as high and i didnt spec out of the nodes and optimized my passive tree for it.

There are also rumors that impending doom does not scale with chaos damage and only with generic spell damage increased. I did not test that so far.

This interaction was present in Poe 1 for whatever it's worth and wasn't even the strongest way to apply Impending Doom. For instance, you can also use spell cascade with cursed ground to get multiple impending dooms off of one cast.

It's almost certainly intended as it's a pretty long-standing interaction. It's also not at all broken (hence almost nobody running it) for a few reasons.

Most importantly, impending doom inherently has less access to support gems. If you think of Impending Doom as a damaging skill, it basically has to use 2-3 supports (cursed ground, enfeeble, and spell cascade) just to be playable, leaving only 2 supports for actually scaling damage whereas most generic skills (say, hexblast for example), can have access to a full 5 support gems.

There's a reason it's not a popular build.

Also, Impending Doom says it does chaos damage - not sure where you heard the rumours about it not scaling with chaos damage. Perhaps you were hearing about how enfeeble (which isn't a chaos spell) would normally inherit the chaos tag in POE1 based on Impending Doom supporting it but this not being the case in POE2? (This is a good thing by the way because of chaos spells damaging you 3% of energy shield and health via a notable in the passive tree).

My point was, why would you ever go for reduced curse duration and impending doom when you can just spam it with cursed ground?

And you can scale archmage to absolute insane levels this way.
As a Gemling int stacker you can reach over 10k mana.
Impending doom has 15% of your current mana as base damage.
You then use archmage and as a gemling, you have even better scaling on it thanks to reduced skill cost and more quality. You can get to 10% of damage as extra lightning per 100 mana you have. With 10k, thats 1000% of base added as lightning.

That means with impending doom you get to 1500 base chaos damage and added 15000 lightning damage. Those are the numbers before any damage increases.

At the same time you can still have tons of ES with The Everlasting Gaze Azure Amulet (30% of mana added as ES). Because ES added as not converted via EB.

You end up with a character with 10k mana and insane damage and tons of ES and Mana for surviveability.

15k base lightning to scale on a spammable curse is not balanced at all.

And at those mana levels you dont need enfeeble. You just use that while leveling because it is the cheapest curse.
Just because it wasnt op in poe1 does not mean its not op now in poe2..

Edit: With Pure Power and Arcane intensity and wildstorm alone you have
~500increased damage and a 1.15 multiplier meaning that those three passives give you over 100k damage/cast.

Where are you even seeing people go for curse duration reduction? That's not a thing except for like one notable on the passive tree where this reduction is featured as a downside in return for increased curse effect. Your point about curse duration reduction being a useless stat is moot - it is supposed to be a useless stat. (In the same way that mana regeneration reduction is a useless stat with the reward being the enabling of Mind Over Matter)

None of this even addresses your original assertion which was that the cursed ground impending doom interaction was unintended. That's for sure wrong - it is intended - it's a longtime mechanic and was a longstanding popular build in PoE1.

But the more interesting discussion is definitely balance where I still think you've missed the mark due to not considering a few things.

Your complaints boil down to mana stacking being too strong which it probably is. However, the interaction that most easily abuses mana stacking is not impending doom but is actually archmage (which applies to all your skills which you can then support with 5 support gems as opposed to impending doom which you can only support with 3 support gems).

Yes, ES is strong, yes Everlasting gaze is strong, yes EB is strong, yes MoM is strong. None of this is exclusive to impending doom. This of course begs the question:

If impending doom is so broken for mana stackers then why is nobody using it?

This comes down to 2 things. Firstly, the giving up of 2 gem support slots just to have impending doom function (enfeeble, cursed ground) is a huge huge DPS loss.

The second thing is perhaps the most important thing of all which is that Impending Doom has a base crit chance of only 5%! This is so incredibly low that with full on critical hit investment, you'd struggle to even hit 40-50% crit chance.

This means that any archmage build using a proper skill with 10-15% crit chance will usually out-DPS an impending doom build, even at the level of 10,000 mana. While impending doom might have a higher base damage than any other skill at 10,000 mana, it might be perhaps 50% or 100% higher than the base hit of another meta skill (keeping in mind that other skills get the benefit of scaling with gem levels up to 27 which impending doom does not).

However, the 50-100% higher base damage does not overcome the massive loss of critical hit which can easily boost damage by 300-400% on not-dogwater-critchance skills

This is not even accounting for the fact that impending doom's clear is terrible without spell cascade and you need to spec heavily into AoE for the clear to feel good unlike skills such as spark whose single target damage scale off of the same stats that boost their clear speed.

It's just not a very good skill at the moment which is why nobody is playing it. Your issues are with mana-stacking which is valid. This indeed does give impending doom a high base hit. However, that high base hit is not enough for it to outperform other archmage-abusing skills. Impending doom loses 2 support gem slots just to function, and has a measly base 5% crit chance, both of which make it a pretty awful skill compared to others. The only ascendancy who could maybe use it properly would be blood mage (due to being able to buff up the base crit to 15%) but then you're playing mana stacking blood mage :l

You basically hit it on the head:

"You end up with a character with 10k mana and insane damage and tons of ES and Mana for surviveability. 15k base lightning to scale on a spammable curse is not balanced at all."

Now just change out "curse" for "spell" and you have the problem. It's Archmage and mana stacking. Not Impending Doom.

That being said, if GGG nerfs mana stacking, I'd like to see them buff other defensive layers such as armour and evasion as the main reason why everyone is turning to energy shield or mana stacking is because those other defensive layers suck and unlike POE1, we don't have the option of converting all our damage to elemental damage.