Stratum Boss is a very powerful kind of monster in the series. As the name suggests, it is often the most powerful monster fought on its home stratum. They are depicted the same way as the FOEs, but instead of a orange/red ball, their color is black. Also, each game has different music for the boss battles. Etrian Odyssey IV and the Untold games instead mark the boss with a purple ring, and occasionally they may come surrounded with a 3x3 purple grid which causes battle to be initiated if the player even enters those tiles.
These monsters, although not always, are most of the time fought on the last floor of said stratum, and will be the last trial the player will face before going further into the labyrinth. They will remain stationary, and the first battle the player will face against said enemy is a mandatory, unnavoidable fight.
Its power is often far bigger than any monster or FOE you faced this far, and often poses a great challenge to the party, often requiring better strategies or bigger preparations by the player.
Another thing that sets these monsters apart from others is their respawn time. While normal FOEs usually take 3 or 7 days, these bosses take 12–15 days to return to life.
Etrian Odyssey BossesEdit
|Fenrir is the Boss of the Emerald Grove. He is the alpha male of the wolf pack that invaded the stratum and been making victims among the new adventurers.||Fenrir|
|Cernunnos||Cernunos is the Boss of the Primitive Jungle. This grotesque creature is the guardian of the stratum, and protects it's deeper reaches.|
|Queen Ant is the first Boss of the Azure Rainforest. She is the queen and leader of the ant army that nests on the upper reaches of this strata.||Queen Ant|
|Cotrangl||Corotrangl is the second Boss of the Azure Rainforest. This gargantuan manta ray lives on the lower floors of the stratum, and is summoned by a mysterious creature, in order to stop the party from advancing any further.|
|Iwaoropenelep is the Boss of the Sandy Barrens. It is guardian of the Forest Folk, and is fought alongside their army.||Iwaopeln|
Etrian Odyssey II: Heroes of Lagaard BossesEdit
|Chimaera is the Boss of the Ancient Forest. He is regarded as the king of the beasts, and made its nest of the upper reaches of said stratum. Its roar can summon beasts from higher stratums to fight at his side.||Chimaera|
|Hellion||Hellion is the Boss of the Auburn Thicket. This powerful demon made the stratum its lair, and has bring death to many explorers in order to protect it from intruders.|
|Scylla is the Boss of the Frozen Grounds. This beautiful beast, despite being extremely powerful, is noted for crying in seemingly endless misery. She seems to have related with the Guild Esbat in some way...||Scylla|
|Harpuia||Harpuia is the Boss of the Petal Bridge. This ancient beast is a deadly nemesis of the Bird Folk, and has preyed upon their kind for ages. They will only open the patch to the next strata if you best this monster.|
|Colossus is the Boss of the Heavenly Keep. He serves as the guardian of the Overlord, and is his last stand agains the explorers. This is one of his most powerful creations.||Colossus|
Etrian Odyssey III: The Drowned City BossesEdit
|Narmer is the Boss of the Waterfall Wood. This catfish lives in the swamp located in the deeps of the said stratum, and the explorers must kill it as a test given by the Senatus. It is extremely cowardly, and will run away from anything at the first opportunity given.||Narmer|
|Ketos||Ketos is the Boss of the Undersea Grotto. This gargantuan, old whale is the guardian of the Deep City, and is loyal to its king.|
|Gatekeeper is the Boss of the Molten Caves. This mechanic soldier protects the lower stratum from outsiders, and keeps whatever lies on the lower floors away from the city.||Gatekeeper|
|Kirin||Kirin is one of the two possible Bosses of the Abyssal Shrine, depending of with side the player has chosen to fight for. This powerful dragon is summoned to stop your advance deeper into the labyrinth. It has powerful Volt and Fire attacks.|
|Shin is one of the two possible Bosses of the Abyssal Shrine, depending of with side the player has chosen to fight for. This mysterious monster, originary from the eastern lands, is summoned to stop your advance deeper into the labyrinth. She takes form of a pale skinned woman, and uses Ice elemental attacks.||Shin|
Etrian Odyssey IV: Legends of the Titan BossesEdit
|Berserker King is the Boss of the Lush Woodlands. This bear is located in the lower reaches of the labyrinth, where he resides as patriarch. It's straight forward and the lower his health gets, the harder he hits.||Berserker King|
|Hollow Queen||Hollow Queen is the Boss of the Misty Ravine. Like the other Hollows before her, she has incredibly high evasion, and will summon other Hollows to assist her in battle.|
|Boiling Lizard is the Boss of the Golden Lair. It is responsible for the clusters of scales that have been heating the floors of the strata, and is incredibly powerful unless the player maneuvers around it to extinguish its scale, which will dramatically weaken it. Mid-fight, it can also shed more of its scales which can amplify the fire damage dealt.||Boiling Lizard|
|Cradle Guardian||Cradle Guardian is the Boss of the Echoing Library. Summoned by the Imperial Prince to halt the party's progress, it has immense physical strength and can change its plating to alter its vulnerabilities to attacks. If defeated, its head can remain and prepare a self-destruct attack.|
Etrian Odyssey Untold: The Millennium Girl BossesEdit
Queen Ant is the Azure Rainforest's first boss monster. She is the source of the ant eggs, and uses her powerful scythe arms to strike down foes.
|Iwaoropenelep is the Forest Folk's Guardian bird of the Sandy Barrens. Its large wingspan and hard beak are not to be taken lightly.||
- Every one of the first stratum bosses of the series (Fenrir, Chimaera, Narmer, and Berserker King) use minions of some kind to help them in battle.
- Cotrangl is the only boss of the original games who doesn't use any minions in battle.
- Azure Rainforest is the only stratum on the series that has two different stratum bosses on it. Although Abyssal Shrine also has two bosses, you can only fight one of them per playthrough.
- You have to fight the Yggdrasil Core (the boss of Claret Hollows) twice in Story Mode before the postgame: the first time with the Yggdrs Virus recieved from Visil, the second with Kupala's aid.
- Oceanmaster Ketos is the first Stratum Boss to have the sentience of a person instead of that of an animal. In fact, he might be the only boss with that criteria (other than possibly the Hollow Queen, but her case is unknown).
- In Etrian Odyssey 2 Untold, the Holy Grail has been confirmed to give consumers of the Elixir regeneration powers so strong that they revive several days after their death, no matter how badly they are wounded; this references the respawning of its Stratum Bosses (the game also states that adventurers are having problems with the Hellion, since she's been said to keep coming back). The respawning is also referenced in Etrian Odyssey 3 (if the player sides with the Deep City); immediately after destroying the Gatekeeper and completing (technically failing) the mission to prevent the Senatus's forces from reaching the 4th Stratum, if you talk to Seyfried before advancing further into the Stratum, he will state that Armoroad's Yggdrasil Tree will create a new Gatekeeper later on.