A lot of this overview's gonna be mostly reappropriated from the EO5 disable post.

Mechanics: Ailments and Binds

Ailments and binds have been a key part of the Etrian Odyssey series from the very beginning, and EOU is no exception. Ailments and binds can very easily swing fights--even a single bind on a single party member or enemy can be the difference between victory and loss. That can be said for ailments in most RPGs, though. What makes EO different is that it doesn't make it impractical and/or impossible for the player to inflict disables on bosses and FOEs; in fact, disables are often the preferred method of making said enemies considerably easier to deal with, and a party that's completely devoid of being able to effectively inflict disables is at a massive disadvantage compared to the opposite.

As for infliction mechanics, they're relatively simple. Every enemy has a set of vulnerabilities to disables, much like they do for damage. Remember that "vulnerabilities" means "multipliers," effectively--an enemy with 150% vulnerability to paralysis, for example, multiplies paralysis infliction chances by 1.5x. A 0% vulnerability, on the other hand, means "complete immunity." For stats used in calculating infliction chances, EOU takes after EO3/4 and uses TEC and LUC. However, unlike those two games, TEC is the primary stat used for determining attacker score, with LUC becoming the primary stat used for determining defense score. This effectively means the best way to boost your party's infliction chances is to boost their TEC.

Each time an ailment/bind is inflicted, that enemy gains 50% vulnerability to that given bind/ailment for 8 turns, which gradually wears off by 7% each turn--a mechanic known as "accumulative vulnerability." Accumulative vulnerability was introduced in EO3 as a way to compensate for ailments/binds becoming much easier to inflict compared to EO1/2. Accumulative vulnerability basically means that it becomes harder, if not impossible, to chain-disable a given enemy, and if you inflict a disable they're innately really resistant to (usually 30%), you just straight-up can't inflict that disable again until the accumulative vulnerability runs out.

Now that that's out of the way, let's discuss the actual ailments and binds themselves! These are organized based on the ailment hierarchy, from lowest to highest. An ailment that's higher on the hierarchy cannot be overwritten with one that's lower. Binds are exempt from this hierarchy altogether. Enemy skill lists mean skills that can be used by the player on Grimoires.


Blind: Blinded entities have their accuracy multiplied by 0.33x, and their evasion completely disabled when not being attacked by other blinded entities.
Blind is one of the more unreliable and, dare I say it, boring disables. Lowering accuracy and disabling evasion is nice and all, but it's just not exciting. Also, it's unreliable, and it's almost always a serious mistake to assume that a blinded enemy can't still be a threat. On the other hand, having a party member be blinded pretty effectively shuts them down, since you should always assume a blinded party member will miss.


Poison: Poisoned entities take damage at the end of every turn. The damage is based on the skill's base damage, and scales based on caster TEC. Poison damage cannot be reduced. The poison damage formula is:
Poison is almost always far deadlier to our party than it is for enemies, although that mostly depends on which poison skill you're using and what enemies you're using it on. Overall, though, Viper's no question the best poison skill, and it can incredibly easy cheese the 1st Stratum boss, and make the 2nd Stratum boss considerably easier.


Paralysis: Paralyzed entities have a 50% chance to not act on a given turn. Paralyzed entities have their evasion disabled.
Paralysis is unreliable now that we don't have the "no action chance is now 99%" food from EO2U. It's still decent, don't get me wrong, and it's not too heavily resisted by EOU's monster roster, but don't plan on the enemy losing all of their actions until it wears off. Paralysis when inflicted on your party members is a slightly bigger deal, since that can seriously screw with your action economy, and a support losing their turn can be a major problem.


Panic: Panicked entities will only perform normal attacks. The attacks can be on opposing entities, allies, or themself.
Panic is extremely dangerous for both the player and the enemy, for reasons I really shouldn't have to state. The only real difference is that panicked enemies, depending on how hard they hit normally, are still very big dangers, just less big dangers than before.


Sleep: Asleep entities will not do anything. Taking damage will dispel the ailment. If the damage is STR-based, it will be multiplied by 1.5x.
Sleep is pretty dangerous for both the player and the enemy, although quite a bit more for the player. Not only do asleep targets lose one or more turns, but taking amplified damage from STR-based sources can very easily kill a player character. For enemies, sleep's effectiveness mostly comes down to "how quickly does the sleep inflictor act", since you, fairly obviously, want to take advantage of the bonus damage. Sleep inflictors that act too fast basically just make enemies waste one turn, which isn't that great.


Curse: Cursed entities will take half of any damage they do as backlash damage. The backlash damage cannot be resisted, but also counts as the original damage type (for conditional drop purposes). If a cursed entity kills someone, they do not take backlash from the fatal damage.
Curse is almost useless for the player due to how much damage enemies have to do kill us--half of that ends up being a pittance. Curse for the enemy, however, can SERIOUSLY fuck us over, especially if it happens early in the turn and you queued up damage skills on damage dealers.


Fear: Afraid entities have a 50% chance to not act on a given turn. If an afraid player character fails to act, they lose TP.
Fear's about as annoying to get by as paralysis when enemies are using it against us. When we're inflicting fear on enemies, though, it's a bit of a special ailment, because Hexers have access to three skills that control afraid enemies: Muting Word, Conflict Word, and Suicide Word. Muting Word and Suicide Word being so powerful arguably makes fear the most powerful ailment the player has access to.


Petrification: Petrified player characters cannot act at all, and take half damage from STR-based sources. Petrified enemies are, effectively, instantly killed. All bosses in the game have 1% vulnerability to instant death.
Petrification is instant death that, in the player's case, can be cured with Refresh or Theriaca Bs. Yawn.


Death: It's...death. Do I really need to explain it? Instant death is a special type of "disable"--it still uses the ailment infliction formula, but has no place in the hierarchy, and will instantly kill an entity, regardless of their HP or endure effects. All bosses in the game have 1% vulnerability to instant death.
Instant death is near useless for the player due to the lack of reliable sources of it. Beheading Cut gets an okay base chance, but it's single-target, and it's stuck on Ronin by default, which has abysmal TEC and LUC.


Stun: Stun is a special disable. It has no place in the hierarchy, and only lasts for one turn. Enemies that are stunned before they take their turn do not act. Enemies that are stunned after they take their turn, effectively, are not affected.
Stun is...interesting. Its effectiveness depends entirely on how quickly the user can act before their target. For enemies, anything they do that can stun usually has some stupidly high speed modifier. Stunning Smash and Caduceus end up with higher speed modifiers, and Sagittarius Shot, Staggering Word, and Shell Shock all go at the start of the turn (well, battle, in Staggering Word's case), as far as player skills go.

Inflicting stun is another problem, though, since most bosses have 10% vulnerability to it.


Head bind: Targets that have their heads bound cannot use skills that require the head. Halves the bound entity's TEC.
Head binds affect basically all TEC-based attacks, as well as some support skills for the player. Head binds are usually incredibly crucial for some bosses, and quite a few FOEs, since a lot of dangerous buffs/TEC-based attacks use the head. Halving TEC also buffs up TEC-based damage considerably--something to keep in mind if you're running with an Alchemist.


Arm bind: Targets that have their arms bound cannot use skills that require the arms. Halves the bound entity's STR.
Arm binds are quite a bit more dangerous for the player than they were in previous games, due to the fact that they shut down every damage dealer (except for Alchemists). They're about the same for bosses as before--that is, arm binds shut early bosses down really hard, but lategame bosses start having a lot more head-based skills while still having some dangerous arm-based ones.


Leg bind: Targets that have their legs bound cannot use skills that require the legs. Halves the bound entity's AGI. Disables the bound entity's ability to escape.
Leg binds are slightly more dangerous than they were in previous EOs due to the fact that they now just full-on reduce AGI (meaning that they affect accuracy as well as turn order), but there's still the big problem of "very few skills for both the player and the enemy use the legs."

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