6 lines to get this working...

Common/src/main/java/at/petrak/hexcasting/api/casting/eval/CastingEnvironment.java

@@ -10,6 +10,7 @@
 import at.petrak.hexcasting.api.pigment.FrozenPigment;
 import at.petrak.hexcasting.api.utils.HexUtils;
 import net.minecraft.core.BlockPos;
+import net.minecraft.nbt.CompoundTag;
 import net.minecraft.network.chat.Component;
 import net.minecraft.resources.ResourceLocation;
 import net.minecraft.server.level.ServerLevel;
@@ -27,6 +28,7 @@
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.function.BiConsumer;
 import java.util.function.Consumer;
 import java.util.function.Predicate;
 
@@ -42,21 +44,21 @@ public abstract class CastingEnvironment {
     /**
      * Stores all listeners that should be notified whenever a CastingEnvironment is initialised.
      */
-    private static final List<Consumer<CastingEnvironment>> createEventListeners = new ArrayList<>();
+    private static final List<BiConsumer<CastingEnvironment, CompoundTag>> createEventListeners = new ArrayList<>();
 
     /**
      * Add a listener that will be called whenever a new CastingEnvironment is created.
      */
-    public static void addCreateEventListener(Consumer<CastingEnvironment> listener) {
+    public static void addCreateEventListener(BiConsumer<CastingEnvironment, CompoundTag> listener) {
         createEventListeners.add(listener);
     }
 
     private boolean createEventTriggered = false;
 
-    public final void triggerCreateEvent() {
+    public final void triggerCreateEvent(CompoundTag userData) {
         if (!createEventTriggered) {
             for (var listener : createEventListeners)
-                listener.accept(this);
+                listener.accept(this, userData);
             createEventTriggered = true;
         }
     }

Common/src/main/java/at/petrak/hexcasting/api/casting/eval/vm/CastingVM.kt

@@ -24,7 +24,7 @@ import net.minecraft.server.level.ServerLevel
  */
 class CastingVM(var image: CastingImage, val env: CastingEnvironment) {
     init {
-        env.triggerCreateEvent()
+        env.triggerCreateEvent(image.userData)
     }
 
     /**