diff --git a/data/battle_param/ep1_rare_monster.toml b/data/battle_param/ep1_rare_monster.toml
index b7e62c4..b806f9a 100644
--- a/data/battle_param/ep1_rare_monster.toml
+++ b/data/battle_param/ep1_rare_monster.toml
@@ -3,13 +3,13 @@
 # 1/512 = 0.001953125
 
 [[Hildebear]]
-rate = 0.01
+appear_rate = 0.01
 
 [[RagRappy]]
-rate = 0.01
+appear_rate = 0.01
 
 [[PoisonLily]]
-rate = 0.01
+appear_rate = 0.01
 
 [[PofuillySlime]]
-rate = 0.01
\ No newline at end of file
+appear_rate = 0.01
\ No newline at end of file
diff --git a/data/battle_param/ep2_rare_monster.toml b/data/battle_param/ep2_rare_monster.toml
index 2b6f916..948b189 100644
--- a/data/battle_param/ep2_rare_monster.toml
+++ b/data/battle_param/ep2_rare_monster.toml
@@ -3,11 +3,11 @@
 # 1/512 = 0.001953125
 
 [[Hildebear]]
-rate = 0.01
+appear_rate = 0.01
 
 [[RagRappy]]
-rate = 0.01
+appear_rate = 0.01
 
 [[PoisonLily]]
-rate = 0.01
+appear_rate = 0.01
 
diff --git a/data/battle_param/ep4_rare_monster.toml b/data/battle_param/ep4_rare_monster.toml
index 9a4b244..9f807c2 100644
--- a/data/battle_param/ep4_rare_monster.toml
+++ b/data/battle_param/ep4_rare_monster.toml
@@ -3,25 +3,25 @@
 # 1/512 = 0.001953125
 
 [[SandRappyCrater]]
-rate = 0.01
+appear_rate = 0.01
 
 [[ZuCrater]]
-rate = 0.01
+appear_rate = 0.01
 
 [[Dorphon]]
-rate = 0.01
+appear_rate = 0.01
 
 [[SandRappyDesert]]
-rate = 0.01
+appear_rate = 0.01
 
 [[ZuDesert]]
-rate = 0.01
+appear_rate = 0.01
 
 [[MerissaA]]
-rate = 0.01
+appear_rate = 0.01
 
 [[Shambertin]]
-rate = 0.1
+appear_rate = 0.1
 
 [[SaintMillion]]
-rate = 0.1
\ No newline at end of file
+appear_rate = 0.1
\ No newline at end of file
diff --git a/src/ship/drops/mod.rs b/src/ship/drops/mod.rs
index 8de0408..4ada11e 100644
--- a/src/ship/drops/mod.rs
+++ b/src/ship/drops/mod.rs
@@ -55,6 +55,17 @@ pub fn load_data_file<T: serde::de::DeserializeOwned>(episode: Episode, difficul
     toml::from_str::<T>(s.as_str()).unwrap()
 }
 
+// this is just copypaste
+pub fn load_rare_monster_file<T: serde::de::DeserializeOwned>(episode: Episode) -> T {
+    let mut path = PathBuf::from("data/battle_param/");
+    path.push(episode.to_string().to_lowercase());
+    path.push("_rare_monster.toml");
+    
+    let mut f = File::open(path).unwrap();
+    let mut s = String::new();
+    f.read_to_string(&mut s);
+    toml::from_str::<T>(s.as_str()).unwrap()
+}
 
 #[derive(Debug, Serialize, Deserialize, Copy, Clone)]
 pub enum MonsterDropType {
diff --git a/src/ship/map/enemy.rs b/src/ship/map/enemy.rs
index 5f9a60e..62c2155 100644
--- a/src/ship/map/enemy.rs
+++ b/src/ship/map/enemy.rs
@@ -1,5 +1,6 @@
 // TOOD: `pub(super) for most of these?`
 use std::io::{Read};
+use std::collections::HashMap;
 
 use byteorder::{LittleEndian, ReadBytesExt};
 use thiserror::Error;
@@ -9,6 +10,10 @@ use crate::ship::room::Episode;
 
 use crate::ship::map::*;
 
+use rand::{Rng, SeedableRng};
+use serde::{Serialize, Deserialize};
+use crate::ship::drops::{load_rare_monster_file};
+
 #[derive(Debug, Copy, Clone)]
 pub struct RawMapEnemy {
     id: u32,
@@ -70,6 +75,46 @@ pub enum MapEnemyError {
     MapAreaError(#[from] MapAreaError),
 }
 
+#[derive(Debug, Serialize, Deserialize)]
+pub struct RareMonsterAppearRate(pub f32);
+
+// making this `pub type` doesn't allow `impl`s to be defined?
+#[derive(Debug, Serialize, Deserialize)]
+pub struct RareMonsterAppearTable {
+    appear_rate: HashMap<MonsterType, RareMonsterAppearRate>,
+    seed: u32,
+}
+
+impl RareMonsterAppearTable {
+    pub fn new(episode: Episode, room_seed: u32) -> RareMonsterAppearTable {
+        let cfg: HashMap<String, f32> = load_rare_monster_file(episode);
+
+        let appear_rates: HashMap<MonsterType, RareMonsterAppearRate> = cfg
+            .into_iter()
+            .map(|(monster, appear_rate)| {
+                let monster: MonsterType = monster.parse().unwrap();
+                let appear_rate = RareMonsterAppearRate(appear_rate);
+                (monster, appear_rate)
+            })
+            .collect();
+
+        RareMonsterAppearTable {
+            appear_rate: appear_rates,
+            seed: room_seed,
+        }
+    }
+
+    pub fn roll_appearance(&self, monster: &MonsterType) -> bool {
+        let mut rng = rand_chacha::ChaChaRng::seed_from_u64(self.seed as u64);
+        if rng.gen::<f32>() < self.appear_rate.get(monster).unwrap_or(&RareMonsterAppearRate(0.0f32)).0 {
+            true
+        }
+        else {
+            false
+        }
+    }
+}
+
 
 #[derive(Debug, Copy, Clone)]
 pub struct MapEnemy {