Precasia habitats: height gates and special spawn branches

Minecraft 1.21.1 · AoA 3.7.16.1. Use this alongside the Precasia Realmstone route and first-portal preparation. It covers seven monster spawn-rule paths, not every creature or encounter in Precasia.

Plan an expedition, not just a biome search

Before entering, prepare a return route, food, spare equipment and free inventory space. In the bestiary, choose 1.21.1 and a creature, then expand Spawn checks · released 1.21.1 defaults. Read its biome list and spawn pool together with the conditions.

The listed seven use the day-or-night monster helper: non-Peaceful difficulty, the dimension's monster block-light limit and a valid spawn block. It does not impose the separate day-phase predicate used by the Overworld day-monster profile. Their additional gates can still reject an attempt.

The diagram shows only height-related branches. It is cropped to Y−20 through 80; applicable bands continue outside that plotted range. A shaded band does not guarantee a spawn, and an unshaded band does not make a place safe from other creatures or existing mobs.

Quick reference

Creature	Extra height/custom gate in Precasia	Extra random input	Biome entries found
Attercopus	Two branches split at Y=50; see below	0.05, or 0.05 × effective difficulty by branch	caves, desert, forest, jungle, ocean, plains, river, savannah, shore, tall forest
Dunkleosteus	Y ≤ 55	0.05 × effective difficulty	ocean
Meganeuropsis	Y ≥ 65	0.05 × effective difficulty	jungle
Scolopendis	Y ≤ 50	1 in 40	caves
Smilodon	Y ≥ 60	0.1 × effective difficulty	forest, jungle, plains, savannah, tall forest
Spinoledon	Y ≥ 60 OR Y ≤ −13	0.05 × effective difficulty	caves, forest, jungle, plains, savannah, tall forest
Veloraptor	Y ≥ 60	0.1 × effective difficulty	desert, forest, jungle, plains, savannah, tall forest

The ordinary minimum/maximum helper bounds are inclusive and apply to its natural-spawn category in the specified dimension. The two custom branches below need separate interpretation. Random inputs are rejection thresholds, not overall encounter percentages. Spawn-pool weight and group size remain available in each card's exact spawn declarations.

Attercopus: the same biome list can hide very different gates

In the custom predicate:

• Outside Precasia: this custom predicate returns true. That alone does not create a natural spawn entry or bypass the common helper.
• In Precasia at Y ≤ 50: a non-natural reason passes this custom branch; a helper-classified natural attempt requires a random float below 0.05.
• In Precasia above Y 50: the biome must be Precasian Desert, sky-darkening must be at least 4, and the random value must be below 0.05 × local effective difficulty. This upper branch does not have the lower branch's natural-reason bypass.

Examples of the custom gate alone:

Context	Result
Precasia, Y 50, natural reason, random 0.049	Pass
Precasia, Y 50, natural reason, random 0.05	Fail at the stated threshold
Precasia, Y 50, non-natural reason	Pass this custom branch
Precasia, Y 51, non-desert biome	Fail, even for a non-natural reason
Precasia, Y 51, desert, sky-darkening 3	Fail
Precasia, Y 51, desert, sky-darkening 4, difficulty 1, random 0.049	Pass this custom branch

This explains why a biome loot/spawn resource listing Attercopus does not prove an unrestricted surface population there. These are source-value predicate examples, not executed spawning tests or a clock-time conversion for sky-darkening.

Spinoledon: two separate altitude bands

Its custom condition accepts Y ≥ 60 OR Y ≤ −13 in Precasia. Thus −13 and 60 pass; −12 through 59 fail that height condition. It does not call the helper's natural-spawn test inside this custom height condition, so do not describe it as automatically bypassed by every non-natural reason.

A separate difficulty-scaled chance gate still comes earlier in its builder. Outside Precasia, the dimension-dependent custom condition passes, but that does not establish natural spawning in another dimension. Commands, spawners and other mods can use different pathways.

Remaining boundaries

• Dunkleosteus: ordinary maximum-height helper at 55, plus difficulty factor 0.05; the inspected biome entry is Precasian Ocean. Do not infer exact underwater depth from its name or the height value alone.
• Meganeuropsis: minimum 65, difficulty factor 0.05; inspected entry is Precasian Jungle.
• Scolopendis: maximum 50 and random threshold 0.025, shown as 1 in 40; inspected entry is Precasian Caves.
• Smilodon and Veloraptor: minimum 60, difficulty factor 0.1; their biome lists differ.

The helper's natural category includes NATURAL, CHUNK_GENERATION, REINFORCEMENT, STRUCTURE and PATROL in the pinned source. Its ordinary dimension-scoped height/chance predicates differ from the explicit custom conditions above. Read the common helper explanation.

Evidence and boundaries

• Seven released class selections, ten biome resources, 320 Attercopus cases and 28 Spinoledon cases
• Illustration source hashes
• Bestiary · Beginner walkthrough

The arithmetic cases isolate custom predicates and do not execute Minecraft classes. They do not establish spawn frequency, exact collision/fluid placement, mob movement, AI safety or modpack compatibility. Creatures without these seven inspected paths, including special/event encounters, are not assigned invented natural-spawn rules. AoA portraits: Tslat and contributors.