diff --git a/src/actor.h b/src/actor.h
index 18d96a0dd7..49d438be8b 100644
--- a/src/actor.h
+++ b/src/actor.h
@@ -326,6 +326,7 @@ enum
 	MF6_ADDITIVEPOISONDURATION	= 0x00200000,
 	MF6_NOMENU			= 0x00400000,	// Player class should not appear in the class selection menu.
 	MF6_BOSSCUBE		= 0x00800000,	// Actor spawned by A_BrainSpit, flagged for timefreeze reasons.
+	MF6_SEEINVISIBLE	= 0x01000000,	// Monsters can see invisible player.
 
 // --- mobj.renderflags ---
 
diff --git a/src/p_enemy.cpp b/src/p_enemy.cpp
index 0931c4d516..19d32626b6 100644
--- a/src/p_enemy.cpp
+++ b/src/p_enemy.cpp
@@ -1667,19 +1667,24 @@ bool P_LookForPlayers (AActor *actor, INTBOOL allaround, FLookExParams *params)
 				continue;
 		}
 
-		if ((player->mo->flags & MF_SHADOW && !(i_compatflags & COMPATF_INVISIBILITY)) ||
-			player->mo->flags3 & MF3_GHOST)
+		// [RC] Well, let's let special monsters with this flag active be able to see
+		// the player then, eh?
+		if(!(actor->flags & MF6_SEEINVISIBLE)) 
 		{
-			if ((P_AproxDistance (player->mo->x - actor->x,
-					player->mo->y - actor->y) > 2*MELEERANGE)
-				&& P_AproxDistance (player->mo->velx, player->mo->vely)
-				< 5*FRACUNIT)
-			{ // Player is sneaking - can't detect
-				return false;
-			}
-			if (pr_lookforplayers() < 225)
-			{ // Player isn't sneaking, but still didn't detect
-				return false;
+			if ((player->mo->flags & MF_SHADOW && !(i_compatflags & COMPATF_INVISIBILITY)) ||
+				player->mo->flags3 & MF3_GHOST)
+			{
+				if ((P_AproxDistance (player->mo->x - actor->x,
+						player->mo->y - actor->y) > 2*MELEERANGE)
+					&& P_AproxDistance (player->mo->velx, player->mo->vely)
+					< 5*FRACUNIT)
+				{ // Player is sneaking - can't detect
+					return false;
+				}
+				if (pr_lookforplayers() < 225)
+				{ // Player isn't sneaking, but still didn't detect
+					return false;
+				}
 			}
 		}
 		
@@ -2757,7 +2762,7 @@ void A_Face (AActor *self, AActor *other, angle_t max_turn)
 	}
 
 	// This will never work well if the turn angle is limited.
-	if (max_turn == 0 && (self->angle == other_angle) && other->flags & MF_SHADOW)
+	if (max_turn == 0 && (self->angle == other_angle) && other->flags & MF_SHADOW && !(self->flags6 & MF6_SEEINVISIBLE) )
     {
 		self->angle += pr_facetarget.Random2() << 21;
     }
@@ -2846,10 +2851,10 @@ DEFINE_ACTION_FUNCTION(AActor, A_MonsterRail)
 									self->target->x - self->target->velx * 3,
 									self->target->y - self->target->vely * 3);
 
-	if (self->target->flags & MF_SHADOW)
-    {
+	if (self->target->flags & MF_SHADOW && !(self->flags6 & MF6_SEEINVISIBLE))
+	{
 		self->angle += pr_railface.Random2() << 21;
-    }
+	}
 
 	P_RailAttack (self, self->GetMissileDamage (0, 1), 0);
 	self->pitch = saved_pitch;
diff --git a/src/p_mobj.cpp b/src/p_mobj.cpp
index bc751e475d..7c431834a6 100644
--- a/src/p_mobj.cpp
+++ b/src/p_mobj.cpp
@@ -5071,7 +5071,8 @@ AActor *P_SpawnMissileXYZ (fixed_t x, fixed_t y, fixed_t z,
 	th->velz = (fixed_t)(velocity.Z);
 
 	// invisible target: rotate velocity vector in 2D
-	if (dest->flags & MF_SHADOW)
+	// [RC] Now monsters can aim at invisible player as if they were fully visible.
+	if (dest->flags & MF_SHADOW && !(source->flags6 & MF6_SEEINVISIBLE))
 	{
 		angle_t an = pr_spawnmissile.Random2 () << 20;
 		an >>= ANGLETOFINESHIFT;
diff --git a/src/thingdef/thingdef_data.cpp b/src/thingdef/thingdef_data.cpp
index 5347e6a1c5..3c0c99c47f 100644
--- a/src/thingdef/thingdef_data.cpp
+++ b/src/thingdef/thingdef_data.cpp
@@ -229,6 +229,7 @@ static FFlagDef ActorFlags[]=
 	DEFINE_FLAG(MF6, ADDITIVEPOISONDURATION, AActor, flags6),
 	DEFINE_FLAG(MF6, BLOCKEDBYSOLIDACTORS, AActor, flags6),
 	DEFINE_FLAG(MF6, NOMENU, AActor, flags6),
+	DEFINE_FLAG(MF6, SEEINVISIBLE, AActor, flags6),
 
 	// Effect flags
 	DEFINE_FLAG(FX, VISIBILITYPULSE, AActor, effects),