Degenerative -Max HP Question

Does this Interface power work on EB, AV, GM and Hamidon/mito critters? I recall that there was a bug in beta where it did not, but I cannot find if it had been fixed.

Thanks for your answer.

i believe it is a % that it is decreased by, but it caps out on AVs, GMs and the like

i think its -10 or -15% max hp normally and against AVs, GMs, and stuff its a set value of -150 max hp per stack so it caps at -600 hp or so

The -150 per stack limit was raised a while ago, to I think 1000 or something. So it's significantly less awful against AVs and above than it originally was.

Claws hits the nail on the head. It makes it a great choice for people who want to solo AVs/GMs on characters who don't have access to absurd amounts of +damage/-resistance/-Regen

Toxic damage is pretty awesome too.

Yes, I do. I'd be particularly interested in the case of a thing with 30000 HP, 50% relevant damage resistance, and 500% regen.

Specifying the target's statistics isn't particularly necessary for discussion since we're not comparing specific powers ("degenerative versus reactive"), but general effect types ("hp debuff versus res debuff"). I will include your numbers as an example, though. Why 500% regen?

I'll edit the explanation into this post if nobody replies first; gimme a bit to compose it.
Wall O' Text Edit:
First, it's important to realize how changes to max hp affect current hp. The short version is "changes to max HP are invisible unless you mouse over the health bar". That is, the bar doesn't move - if you were at half health before the change, you'll be at half health after. Half of the new HP total is a different number than half of the old total, of course, but whatever fraction of max health the target is at before the hp change will be preserved through the change. You can observe this with powers like One with the Shield that change maxhp but do not heal; it happens with powers like Dull Pain as well, but it's harder to tell because DP also has a large heal component. This behavior is the same whether it's an increase or decrease, an effect being applied or an effect wearing off.

So, what happens if a -hp debuff is applied to a target that is not taking damage? Nothing. They continue to regenerate HP in 5% ticks at the same rate they did before. Each tick is smaller than without the debuff, though. And if the -hp wears off, each 5% tick they regenerated under the debuff "stretches" to be 5% of their now-higher health total, just as if they had their full non-debuffed regen rate the whole time.
So, if your example AV is at half health (15k/30k), and receives a -1000 HP debuff, it will be at 14.5k/29k. It then regenerates a tick, rising to 55% health (15.95k/29k). If the debuff wears off, it will be at 16.5k/30k.

What if the target takes a 1000-damage hit during the debuff? It starts at 15k/30k, gets debuffed to 14.5k/29k, then gets hit. The hit is reduced to 500 by damage resistance, and the AV is at 14k/29k (48.28%). If the debuff wears off, they will be at the same percentage, which is 14,483 health. The 500-damage hit took away more than 500 health: its damage was amplified by the debuff.

For comparison, what happens if a -regen debuff is applied to a target that is not taking damage? Their regeneration ticks come less often, but each tick is the same amount as it was without the debuff. If the debuff wears off, they don't suddenly get back all the health they would have regenerated without the debuff. A regen debuff is equivalent to some flat amount of extra DPS: -50 hp/sec regeneration is equivalent to dealing 50 DPS for the same duration. And of course, a regen debuff doesn't change the damage dealt during its duration in any way.

So, an hp debuff amplifies damage that lands during its duration, making it easier to power through the target's regeneration, but it does not slow the target's regeneration ticks at all. This would be very strange behavior for a regen debuff, but is very closely analogous to a resist debuff.

The example AV regenerates (30000 hp)*(5% hp/15 seconds)*(500% regen) = 500 hp/second. A full 4 stacks of Degenerative reduces that to (26000 hp)*(5% hp/15 seconds)*(500% regen) = 433.3 hp/second. So, a team that deals 500 DPS will be able to make progress on this AV with the debuff, at a rate of 66.7 hp/sec - it would take 390 seconds to defeat the target (I assume your example is the Cathedral of Pain, so in practice they won't get 390 seconds to wail on the AV, of course). Without the debuff, the team would need to deal 576.92 DPS to take down the AV in that time. The debuff has effectively added +76.92 DPS (15.38%).
What if the team does 1000 DPS? With the debuff, they'd take down the target in 45.88 seconds. Without the debuff, the team would need 1153.85 DPS to do that. The debuff has effectively added +153.85 DPS (15.38%).
2000 DPS? With the debuff, they'd take down the target in 16.60 seconds. Without the debuff, they'd need 2307.69 DPS to do that. The debuff has effectively added +307.69 DPS (15.38%).
In each case, debuffing the target's max health by 13.33% effectively added 15.38% DPS. So, you can see that the debuff effectively amplifies damage dealt, much like a resist debuff, it doesn't just add a flat amount like a regen debuff does. Specifically, a -X% change to max health is equivalent to the target taking [100/(100-x)] times as much damage: 13.33% -hp effectively makes the target take 1.1538x as much damage, or 15.38% more.

Now, there are a few ways in which this analogy does not quite hold:
1. Direct heals. As in, the kind that make green numbers pop up. -hp acts like negative resistance to heals in the same way it acts as negative resistance to damage: a heal for 1000 gives you back more of the bar when it's out of 29k than out of 30k (and if the debuff then wears off, the 1k hp gained on the first target stretches to 1034 hp). Direct heals are pretty common for player characters (every character can use green inspirations, before even mentioning actual powers), but they are relatively rare among NPCs, so this is usually not important for Degenerative.
2. Interaction with actual resist debuffs. Resist debuffs stack additively with each other, but -hp stacks multiplicatively with -res. This can be a big deal if there's lots of other debuffs flying around.
3. Resist debuffs are linear (-80% res is four times as good as -20%), but -hp has above-linear returns: a -20% hp debuff would make the target one-fourth easier to defeat than an un-debuffed foe, but a -80% hp debuff would make the target five times easier to defeat! However, since Degenerative is the only -hp effect around, and doesn't stack very high, and is always fairly small relative to the target's max health, in practice you can't debuff the target enough for this to matter much.

So, yes, -hp definitely makes the target regenerate less hp/sec, as long as the debuff stays up. It's not wrong to say it reduces regen. But it is, IMO, more useful to think of it as an amplifier for the damage the target takes instead. Quantitatively, it's not perfectly equivalent, but qualitatively it is very similar to -res, and not very similar to -regen.

Because it's an easy number to work with and large enough to be illustrative. I didn't have a particular AV in mind. You outlined adequately with your numbered points exactly what I was after (#2 is why I specified the theoretical AV's 'relevant resistance'), so I'll just look at you askance for calling it the same as a -res debuff but know it's just an odd (to me) terminology choice.

I still think telling anyone who doesn't necessarily know how res debuffs work that -hp is 'the same as' a res debuff will just lead to confusion for them down the road, though.

Caveat #2 doesn't actually care about the target's resistances: -13.33% hp and -15.38% res (by themselves) both add +15.38% DPS, regardless of how much resistance the target has to start with, due to the way resistances resist debuffs. It's just the way it combines with other debuffs that differs.

It definitely is potentially confusing, but saying it reduces regen is potentially misleading, so it's kind of a lose-lose on that front

Edit: Pretty much this same discussion was why Degenerative had the -150 limit in the first place, and was subsequently raised to -1000. Arbiter Hawk saw -2% hp per stack as the equivalent of "dealing 2% of their max health, up front", which was too good against targets with high health, but showing that the debuff on its own was much more comparable to a resist buff (-150 HP to a an AV with 26k is comparable to ~.5% -res, which is far worse than Reactive) than a large chunk of up-front damage and -regen is, IIRC, what led to the eventual buff to -1000.