Could potentially be helpful in reversing a bad position by letting you spam Atonement/Soul Transfer, but in general this effect is so niche, and Miasma Tsunami is so good, that in most cases you're probably just going to use this as a free 3-turn duration extender leading up to Tsunami, since that's something you want to time the use of to get the most out of its effect anyways.

The easiest way to use it is to just unleash all your damage breaks alongside Tsunami for a huge burst turn, but it can also be used to inflict something, launch a drive, whatever. The amplifying effect is huge, and if you have a Harbinger in your party, you should definitely come up with a plan to get the best use out of it.

Defense debuff is universally useful. Even though it's weaker than most attack buffs, keeping it on the enemy is also a lot easier than keeping buffs on your own characters. It's a fundamental move that you'll be using from start to finish.

The 10% deficit on attack debuffs compared to defense buffs is a lot more impactful than the 10% deficit on defense debuffs compared to attack buffs (basically, Eroding Miasma is generally around 10% less effective than attack buffs, but Stifling Miasma is always 40% less effective than defense buffs). If your party can't make use of many other debuffs, having it on can't hurt, and it can be occasionally useful to nullify attack buffs (though prereq level is enough for that purpose), but it's generally not a great way to use a debuff slot.

If you have an Imperial or a Gunner, having high levels of this (and some speed tinkering on your Imperial and Gunner) can let them get off drives and charged shots before enemies, which is pretty awesome. It also can sometimes lets you go before counterattacks (getting an elemental attack off before a certain stony asshole prepares to counter them is always a hoot), though that doesn't happen often enough to justify maxing a skill for. The evasion debuff isn't very strong, and you'll need another source to help Ricochet or Helm Splitter to hit reliably without goggles. Conclusion: Get it if you want to support an Imperial/Gunner; keep it at prereq otherwise for anything that tries to use evasion buffs.

While Harbinger kept their status reaps, they're more or less just kinda there. The base rate kinda sucks a lot even when maxed, and Harbinger's stat spread doesn't really help matters (though they do get status ATK up which helps a little). The damage is also pretty bad. Maxing a reap or two gives your Harbinger some AoE damage, which can help with randoms, but maxing all of them out is a heavy SP investment (on a class that's already very strapped for SP) for rather paltry returns, so just pick one or two status you like and get them if you want. Or just skip them altogether because Harbingers already have a million other skills they want to get, most of which make far more impact than what generally amount to inferior versions of gas items that you can spam for TP and has a tiny bit of damage attached.

For this reap in particular, Paralyze is pretty inconsistent, even more so than blind, but it can sometimes stop nasty support moves (unlike blind) and costs the least amount of SP to get. I generally think blind is a bit more useful, but this is not a bad choice either if you must get a status reap.

If you max it out, the poison damage it does is pretty respectible in its own right; it doesn't compare to Caltrops or Poison Circle, but it is more damage than Poison Gas (and is available far earlier). On the other hand, poison does nothing to hinder the enemies' attacks. Also, unlike the other reaps, you really need to max this out to make good use of it, as the poison damage is pretty futile at non-max levels. Still, if I really want a reap attack for randoms, I'd pick this one as my choice just because it's probably the most random-oriented one of the bnuch.

The speed bonus is very important, because the miasmas all have fairly low action speed and needs the bonus to stand a good chance of actually going before your party's attacks. Max it as soon as possible.

The fact that it works with any debuff skills, not just miasma skills, means that it works pretty well with other classes' skills. Keep this in mind for whatever subclass you choose, or if you want to choose Harbinger as another character's subclass.

If you just need the HP healing, it heals for plenty even at a fairly low level. If you want the ailment clear to be reliable, however, then you'll need to max it. This is a good skill, but Harbinger is very strapped for SP, so you'll really want to think about when you're ready to put more points in it.

I actually find the recovery reduction effect to be even more impactful than the infliction increase effect, which is nice since that part works pretty well even at low levels, making these skills a rare, merciful exception to the whole "Harbinger skills need investment to really function" shtick that will plague your Harbinger's existence for a long time. The same quality also makes them pretty nice as subclass skills if your party makes use of inflictions (and doesn't have a Harbinger main class already).

It's a bit more easier to inflict than paralyze, has a greater chance of kicking in, and helps your attacks hit, but it does nothing to hinder support skills, and costs several points of prerequisite whereas Paralyzing Reap costs none (though this is a moot issue if you want Chaos Reap/Fatal Reap as well). If you want a reap that helps out both in randoms and in boss battles, this would probably be a good choice.

As strong as Panic is, the base chance sucks too much for me to want to rely on it to any degree. Still, Panic is strong, so if you really don't want to settle for an inconsistent disable, then go ahead and take it.

If you must have this skill, max it as soon as possible, or it will basically never work; ever.

It's basically guaranteed to be there all the time, which helps with skill spamming greatly, but Harbinger is just so strapped for SP that they'll be hard pressed to find spare ones to throw here. On the other hand, they don't tend to be so strapped for TP unless they're going for Ephemeral Reap. Basically, just get it later unless you want Ephemeral Reap, in which case you'll probably want to sacrifice other skills a bit to get this early because Ephemeral Reap is really, really expensive TP-wise.

It's a fairly nice bit of upkeep, and that's about it. Unfortunately, "a fairly nice bit of upkeep" faces very hard competition with the rest of Harbinger's skillset, and you probably won't have the chance to invest in it until much later.

Same deal with a lot of Harbinger passives. It's nice to have, but not so nice that you'll have the chance to really invest in them until much later.

The damage it deals is sorta OK, though not really something that you can make an effective party out of. It certainly tends to contribute more than trying to inflict something with 40% base chance, and so can make a nice skill of choice on any free turns you get. If you have a frontline slot open, getting it isn't the worst of ideas, though watch out for that TP cost.

It no longer has any sleep bombing application, and is now just a damage skill with instant death attached. 500% isn't a lot of damage for a support class, so it's mostly just something to use for instant death, and I don't really like that 25% base chance. There are some applications of instant death in postgame, though, so you'll want to max it out before then.

If you have an AI writeup for whatever you're fighting in front of you, which you should given that you're reading this LP, then these skills can be a godsend if your party otherwise doesn't have many good ways of recovering from mass inflictions. Unfortunately, these skills aren't very reliable unless you have maxed them out, and they aren't very spammable without Enduring Armor either. You'll need to decide whether you want to sacrifice other skills to get them running as soon as possible, or sacrifice these to focus on more generally useful skills. Then again, that holds true for just about any other Harbinger skills. Isn't playing an SP starved class great? Of the two, I'd say Miasma Wall is the more important one. Debuffs can be cleared away with Unihorns if necessary (and don't prevent skill use/character control), and instant death is really rare. There are some really nasty debuffs in the endgame that are difficult to negate with buffs, however, so you'll want Spirit Barrier eventually.

This makes those support skills so much easier to use it's not even funny. As SP starved as Harbingers tend to be, this one should really be a priority unless you rarely use those skills.