Enemies, usually bosses due to involving more development focus, fought in multiple triggered phases, are usually made with the intent that all those phases are seen and that the triggers ensure it. But some types of games, like those with Absurdly High Level Caps, could get players so strong that bosses are One-Hit Killed, leaving no time for phases to happen.
In that case, the developer has to do this trope, intervening in some way so their creative vision will be properly expressed.
If phases are triggered by an enemy going under certain Hit Points amounts and the creator can't ensure that the enemy can't be beaten too efficiently enough to jump from one phase's range into another range that's not immediately after it, for example, if it's not guaranteed that an enemy that changes every 10% of HP loss, can't have range 8 skipped by a Critical Hit sending them from 95% into 75% HP. If skipping is functionally possible, then there might be a boss-only feature that caps the damage so it's actually impossible, which would be this trope. Functionally, that example is like Multiple Life Bars, but where excess damage from one life bar can't be transmitted to other life bars. But in this case, it's one single Life Meter/set of Hit Points.
Other forms of this trope depend on how the phase guarantee works, but includes things such as changing attack sequences so they do an attack that renders them invulnerable, note Boss Vulnerability, sooner than they should, right before the phase change, or extending the Mercy Invincibility to cover the phase change animation.
Compare Broken Armor Boss Battle, where a boss has some form of defence that must be overcome before the player can damage them properly.
If the trigger is the whole enemy group being defeated and other enemies coming onto the scene, such that it's functionally multiple bosses, but the usual defeat effects are actually swapped out with another enemy taking the field, then it's not this trope, but an enemy wave / Sequential Boss.
A Wolfpack Boss losing a member, Healing Boss-ing up the other members, but not changing tactics, isn't this trope because it's not a different phase if no combat tactics change.
Turns Red overlaps when health loss is the trigger for the tactics change.
Likely to overlap with Boss Vulnerability. Might be a Scripted Battle. May be done by having the enemy work by different rules, a.k.a My Rules Are Not Your Rules.
Waves of Enemies might be disguised as this, if the transformation replaces an enemy's defeat animation, in which case it's this but a Subverted Trope. Compare You Can't Thwart Stage One for more general cases of villainous plans that a story's creators ensure can't be skipped. This Index Is Unstoppable is for characters.
Examples:
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Action-Adventure
- ANNO: Mutationem: In the fight against the Bloodlusters, one of them will be invulnerable to damage as it starts Hulking Out into a second taller form after its initial health meter is depleted.
Action RPG
- Borderlands 3: This game has a segmented boss health bar, where these bosses will usually set up an invulnerable shield and call in allies to attack instead.
- Dark Souls:
- Dark Souls I has Ornstein and Smough, who start out as a Dual Boss. When you kill one of them, the other one absorbs their power to bring themselves back to full health and gain some of their fallen partner's abilities.
- Dark Souls III: The Abyss Watchers, who are fought as a Wolf Pack Boss: During the first phase where the player has to kill them all while they fight amongst themselves. Then, after the strongest Watcher has taken enough damage, a cutscene plays where a single Watcher absorbs the blood of his brothers, wreathing his sword in flame and becoming more of a proper boss.
- Genshin Impact: Players are actively punished for trying to kill Signora too fast, as hitting her with a single sufficiently powerful attack in a bid to One-Hit Kill her will not only negate the surplus damage beyond a set percentage of her max health (30% in her first phase, and 20% in her second) but also deal a heavy damage AoE and fully fills the Sheer Cold/Blazing Heat gauge, causing players to take damage over time.
- Kingdom Hearts II: The game prevents you from skipping boss phases by making the boss invincible at certain phases or using a mechanic that causes them to retaliate or teleport away from you if you hit them a certain amount of times.
- Path of Exile: Many bosses, particularly ones that are or were endgame bosses, have phases where they become invulnerable and/or untargetable as they power up, change abilities, or perform a special attack. In the case of bosses where their challenge hasn't kept up with player Power Creep these periods of invulnerability can compose the majority of the fight's duration, leading to some calls for the fights to be reworked to skip or accelerate them.
Beat-'Em-Up
- Hi-Fi RUSH: The bosses can only go to their next phase when both the music progression and damage taken have passed certain thresholds.
Card Battle Game
- Hearthstone:
- Kel'Thuzad is a Broken Armor Boss Battle, where breaking the armor forces the player's turn to end immediately instead of progressing like it's just a second section of hitpoints. Kel'Thuzad summons a pair of minions and replaces his original hero power (which freezes and damages the player's hero) with a new one that lets him steal the player's minions. If you take too long to break his armor and are somehow not dead, Kel'Thuzad will go into the next phase anyway.
- The Final Boss of the Galakrond's Awakening adventure starts with Rafaam conducting a ritual to summon Galakrond, who the boss fight switches to. If you somehow manage to kill Rafaam before the ritual is complete, he'll become Immune and complete the ritual anyway. The same thing will happen if you try to replace his hero power (how he transforms) with something else.
MMORPG
- Fate/Grand Order: Following the first campaign, bosses will often come with two or more health bars, and breaking each one will result in the boss entering the next phase of the battle. There is no way to by-pass each phase by overdamaging the bar.
- Final Fantasy XIV: At the end of the Aetherochemical Research Facility, you initially fight Lahabrea and Igeyorhm together as a Dual Boss, after which they do a Fusion Dance and become Ascian Prime. Neither of them can actually be defeated prior to the fusion; that last 0.1% of HP will stay there until they're both weak enough to trigger the cutscene.
- RuneScape: The developers have implemented mechanics into several boss fights to ensure that, once bosses hit a certain health threshold, they cannot dip below that threshold until something else is done. For instance:
- Every 40,000 health, Nex requires the player to kill the mage that's currently empowering her in order to move on to the next phase.
- The Magister will stop taking damage for a few game ticks every 50,000 health so that they can do a special attack without being interrupted.
- Kerapac the Bound will rewind time when he reaches a certain health threshold in his first three phases, before he Turns Red and does increasing damage in the fourth phase.
- World of Warcraft: The Iron Maidens in Blackrock Foundry are a three-member Wolfpack Boss that has ways to elongate the fight and phases that trigger both based on how long the battle's been, or whenever any member drops below 20% Hit Points. The latter has all three enrage and unlock all abilities. The only way to prevent any new phases from happening would be to defeat them all before 1 minute passes, with a single strike that takes out more than 20% of their Hit Points. Whether that's possible to achieve is out of the scope of this trope.
Platformer
- Cave Story: Misery will defensively teleport when taking enough damage. This allows her to engage with up to four spinning circles, which provide some additional defense and also convert themselves into an attack when she teleports again.
- Kirby:
- Bosses in Kirby's Return to Dream Land, the Team Kirby Clash series, Kirby Fighters 2 and Kirby's Return to Dream Land Deluxe become invulnerable to damage when transitioning from one phase to the next.
- In main series titles starting with Kirby: Triple Deluxe, the bosses merely gain a sizeable defense boost instead of outright invulnerability, but it's still enough to make it impossible to skip any phases.
- In Kirby and the Forgotten Land, most bosses cap the amount of damage you can do so that it's impossible to defeat them before the phase transition completes when they're normally one more attack away from defeat (Gorimondo is the only exception).
- Rogue Legacy 2: Bosses gain a massive boost to their defense while they Turn Red, preventing the player from just plowing through health before the second phase can start.
Role-Playing Game
- In Another Eden, the increasing power level of characters and increasing complexity of battle systems leads to the the amount of damage the player can do in one turn (with the help of Another Force) or even one attack continuously increasing. To prevent players from skipping past boss phase changes in a single burst, newer superbosses often come with "HP stoppers" which prevent them from taking any further damage the moment they fall to a threshold where the boss Turns Red. Some bosses combine this with Multiple Life Bars.
- The Diablo series:
- Diablo III: Bosses that have multiple stages (such as Belial, Diablo, Urzael, and Malphael) become immune to damage for a short time once the player has done enough damage to trigger the next phase of the fight.
- Belial teleports to a platform where it's impossible for the player to reach him before going One-Winged Angel.
- Diablo and Urzael both have attacks that will always stun the player, which they will always use at the end of the 1st phase to trigger the 2nd.
- In Diablo IV, Lilith's health cannot drop to zero until she has completed her three intermissions. If a player does enough damage to skip one or more intermission, she will instead do the intermissions back-to-back while being immune to damage. This was later applied to Andariel and Duriel as well.
- Diablo III: Bosses that have multiple stages (such as Belial, Diablo, Urzael, and Malphael) become immune to damage for a short time once the player has done enough damage to trigger the next phase of the fight.
- The Final Fantasy series:
- Final Fantasy V Advance, Enuo has two hidden parts that ensure that the second phase of the boss spawns even if you defeated the first phase with an attack that prevents counterattacks (such as Vulner Cannon).
- Final Fantasy VII Remake, and its sequel, Final Fantasy VII Rebirth, often give health thresholds to bosses, that when reached, nullify further damage until the triggered cutscene plays.
- Final Fantasy X: One line of Giant Flyer enemies starts the battle hovering above the ground, then fall to earth when their HP gets too low. In practice, this means they can never be one-shotted and extra damage is wasted because they have a Last Chance Hit Point.
- Golden Sun: The Lost Age: The Doom Dragon is secretly a Sequential Boss, even if there's seemingly no interruption in the battle: Killing one of its heads actually replaces it with the version with one less head (so three phases total) to waste any further damage, as a counter to many players' Alpha Strike strategy of firing off every summon they have to one-shot or two-shot bosses.
- Paper Mario: The Thousand-Year Door: Two boss fights have events that take place during the battle (the first Doopliss fight and the Shadow Queen's first form), but it is possible with the right setup to take them all the way down to 0 health before it triggers. If you do so, they will automatically heal a small amount of HP (10 and 1 respectively) to allow the scene to play out as normal.
- Pokémon Sword and Shield: Higher-tier Max Raid targets put up Deflector Shields multiple times during the raid, at specific HP thresholds. If you manage to reach the next threshold without breaking the previous shield, the Pokémon will stop taking damage entirely until it gets a chance to put up the new shield.
- Octopath Traveler: The main strategy in battle is to "Break" the enemy by striking their weaknesses (which can be certain types of weapons, specific elemental attacks, or both) until their 'shields' are all broken, which will both stun them and lower their defenses for at least one turn. However certain bosses, especially in the later chapters for each character's story, may change their weaknesses and refresh their shields (frequently gaining more as well) after a certain amount of damage is dealt to them (usually signifying a narrative moment), often forcing the player to find out what the new weaknesses are.
- Persona 5: Shadow Sae has two phases. The 65,535 HPnote first phase is for narrative reasons, and not intended as the real boss. You're supposed to expose her cheating, in order to make her go berserk and enter the 8,000 HP second phase. With enough patience, she ''can'' be defeated in her first form, but that would only be good for out-of-universe bragging rights. It doesn't end the battle. Instead, it'll just enter the second phase anyway.
- Recettear: An Item Shop's Tale: The Geddon Unit can only lose at most 1/6 of its total Hit Points at once, before it retreats into nearly untouchable range, destroys some of the floor, then returning into attackable range.
- Trials of Mana: Damage-Sponge Boss Dangaard's fight is split into four phases, one for each quarter of its Hit Points, and damaging it isn't the easiest thing since everyone's status effects are reset to "none" when the phase changes.
Tower Defense
- Tower Defense Simulator does this with two of its bosses, while it's Played With on several others.
- The first one is the Molten Warlord, the final boss of Molten Mode. When it gets to half HP, it temporarily gains the Tank attribute, which puts a damage Cap of 1. This means its second phase is unskippable.
- The second is the Fallen King, specifically the one in Hardcore mode. At half HP, it gains a shield equal to 1/10th of its HP, and is also invulnerable during the transition to its second phase, in which it gets faster and calls in 2 (occasionally 3) Fallen Guardians at the entrance of the map.
- Subverted with the Fallen Swordmaster. Whilst it becomes invulnerable during the transition to its second phase, it doesn't trigger the second phase until when it would next attack after hitting half HP. So with enough DPS, you can skip it entirely.
- Downplayed with the Void Reaver, Gunslinger and Nuclear Monster. They are both vulnerable during their second phase transitions, but they have so much HP that skipping the second phases is essentially impossible without using Admin Mode in Sandbox Mode to buff towers to kill them quick enough.
- Double Subverted for Wox the Fox, the boss of Pizza Party. He, too can be killed during his second phase transition, however he'll still Jump Scare your towers, reducing their range temporarily, even if he's dead 2 seconds after he hits half HP. Actually, his first Jump Scare, which occurs 5 seconds after spawning, cannot be skipped by killing him either, even if you were to One-Hit Kill him.
