diff --git a/PKHeX.Core/Legality/LegalityCheckStrings.cs b/PKHeX.Core/Legality/LegalityCheckStrings.cs
index 5fb9170be..7ce2a44aa 100644
--- a/PKHeX.Core/Legality/LegalityCheckStrings.cs
+++ b/PKHeX.Core/Legality/LegalityCheckStrings.cs
@@ -188,6 +188,10 @@ namespace PKHeX.Core
 
         public static string LFavoriteMarkingUnavailable { get; set; } = "Favorite Marking is not available.";
 
+        public static string LFormArgumentHigh { get; set; } = "Form argument is too high for current form.";
+        public static string LFormArgumentLow { get; set; } = "Form argument is too low for current form.";
+        public static string LFormArgumentNotAllowed { get; set; } = "Form argument is not allowed for this encounter.";
+        public static string LFormArgumentValid { get; set; } = "Form argument is valid.";
         public static string LFormBattle { get; set; } = "Form cannot exist outside of a battle.";
         public static string LFormEternal { get; set; } = "Valid Eternal Flower encounter.";
         public static string LFormEternalInvalid { get; set; } = "Invalid Eternal Flower encounter.";
diff --git a/PKHeX.Core/Legality/Verifiers/FormVerifier.cs b/PKHeX.Core/Legality/Verifiers/FormVerifier.cs
index 131956f8c..ec524dfed 100644
--- a/PKHeX.Core/Legality/Verifiers/FormVerifier.cs
+++ b/PKHeX.Core/Legality/Verifiers/FormVerifier.cs
@@ -18,6 +18,9 @@ namespace PKHeX.Core
                 return; // no forms exist
             var result = VerifyForm(data);
             data.AddLine(result);
+
+            if (pkm is IFormArgument f)
+                data.AddLine(VerifyFormArgument(data, f));
         }
 
         private CheckResult VALID => GetValid(LFormValid);
@@ -250,5 +253,72 @@ namespace PKHeX.Core
                     break;
             }
         }
+
+        private CheckResult VerifyFormArgument(LegalityAnalysis data, IFormArgument f)
+        {
+            var pkm = data.pkm;
+            switch (data.pkm.Species)
+            {
+                default:
+                    {
+                        if (f.FormArgument != 0)
+                            return GetInvalid(LFormArgumentNotAllowed);
+                        break;
+                    }
+                case (int)Species.Furfrou:
+                    {
+                        if (f.FormArgument > 5)
+                            return GetInvalid(LFormArgumentHigh);
+                        if (f.FormArgument == 0 && pkm.AltForm != 0)
+                            return GetInvalid(LFormArgumentNotAllowed);
+                        if (f.FormArgument != 0 && pkm.AltForm == 0)
+                            return GetInvalid(LFormArgumentNotAllowed);
+                        break;
+                    }
+                case (int)Species.Hoopa:
+                    {
+                        if (f.FormArgument > 3)
+                            return GetInvalid(LFormArgumentHigh);
+                        if (f.FormArgument == 0 && pkm.AltForm != 0)
+                            return GetInvalid(LFormArgumentNotAllowed);
+                        if (f.FormArgument != 0 && pkm.AltForm == 0)
+                            return GetInvalid(LFormArgumentNotAllowed);
+                        break;
+                    }
+                case (int)Species.Runerigus:
+                    {
+                        if (data.EncounterMatch.Species == (int)Species.Runerigus)
+                        {
+                            if (f.FormArgument != 0)
+                                return GetInvalid(LFormArgumentNotAllowed);
+                        }
+                        else
+                        {
+                            if (f.FormArgument < 49)
+                                return GetInvalid(LFormArgumentLow);
+                            if (f.FormArgument > 320) // calculate Yamask HP maximum? Arbitrary bound check.
+                                return GetInvalid(LFormArgumentHigh);
+                        }
+                        break;
+                    }
+
+                case (int)Species.Alcremie:
+                    {
+                        if (data.EncounterMatch.Species == (int)Species.Alcremie)
+                        {
+                            if (f.FormArgument != 0)
+                                return GetInvalid(LFormArgumentNotAllowed);
+                        }
+                        else
+                        {
+                            if (f.FormArgument > (uint)AlcremieDecoration.Ribbon)
+                                return GetInvalid(LFormArgumentHigh);
+                        }
+                        break;
+                    }
+            }
+
+            return GetValid(LFormArgumentValid);
+        }
     }
 }