PaperMC/paper-server/patches/sources/net/minecraft/world/effect/MobEffectUtil.java.patch

--- a/net/minecraft/world/effect/MobEffectUtil.java
+++ b/net/minecraft/world/effect/MobEffectUtil.java
@@ -47,18 +_,31 @@
     public static List<ServerPlayer> addEffectToPlayersAround(
         ServerLevel level, @Nullable Entity source, Vec3 pos, double radius, MobEffectInstance effect, int duration
     ) {
+        // CraftBukkit start
+        return MobEffectUtil.addEffectToPlayersAround(level, source, pos, radius, effect, duration, org.bukkit.event.entity.EntityPotionEffectEvent.Cause.UNKNOWN);
+    }
+
+    public static List<ServerPlayer> addEffectToPlayersAround(ServerLevel level, @Nullable Entity source, Vec3 pos, double radius, MobEffectInstance effect, int duration, org.bukkit.event.entity.EntityPotionEffectEvent.Cause cause) {
+        // Paper start - Add ElderGuardianAppearanceEvent
+        return addEffectToPlayersAround(level, source, pos, radius, effect, duration, cause, null);
+    }
+
+    public static List<ServerPlayer> addEffectToPlayersAround(ServerLevel level, @Nullable Entity source, Vec3 pos, double radius, MobEffectInstance effect, int duration, org.bukkit.event.entity.EntityPotionEffectEvent.Cause cause, @Nullable java.util.function.Predicate<ServerPlayer> playerPredicate) {
+        // Paper end - Add ElderGuardianAppearanceEvent
+        // CraftBukkit end
         Holder<MobEffect> effect1 = effect.getEffect();
-        List<ServerPlayer> players = level.getPlayers(
-            serverPlayer -> serverPlayer.gameMode.isSurvival()
-                && (source == null || !source.isAlliedTo(serverPlayer))
-                && pos.closerThan(serverPlayer.position(), radius)
-                && (
-                    !serverPlayer.hasEffect(effect1)
-                        || serverPlayer.getEffect(effect1).getAmplifier() < effect.getAmplifier()
-                        || serverPlayer.getEffect(effect1).endsWithin(duration - 1)
-                )
-        );
-        players.forEach(serverPlayer -> serverPlayer.addEffect(new MobEffectInstance(effect), source));
+        List<ServerPlayer> players = level.getPlayers((entityplayer) -> {
+            // Paper start - Add ElderGuardianAppearanceEvent
+            boolean condition = entityplayer.gameMode.isSurvival() && (source == null || !source.isAlliedTo((Entity) entityplayer)) && pos.closerThan(entityplayer.position(), radius) && (!entityplayer.hasEffect(effect1) || entityplayer.getEffect(effect1).getAmplifier() < effect.getAmplifier() || entityplayer.getEffect(effect1).endsWithin(duration - 1));
+            if (condition) {
+                return playerPredicate == null || playerPredicate.test(entityplayer); // Only test the player AFTER it is true
+            } else {
+                return false;
+            }
+            // Paper end - Add ElderGuardianAppearanceEvent
+        });
+
+        players.forEach(serverPlayer -> serverPlayer.addEffect(new MobEffectInstance(effect), source, cause)); // CraftBukkit
         return players;
     }
 }