For v2.2.

api/pom.xml

@@ -8,7 +8,7 @@
     <parent>
         <artifactId>npclib</artifactId>
         <groupId>net.jitse</groupId>
-        <version>2.1-SNAPSHOT</version>
+        <version>2.2-SNAPSHOT</version>
     </parent>
 
     <artifactId>npclib-api</artifactId>

api/src/main/java/net/jitse/npclib/internal/SimpleNPC.java

@@ -284,6 +284,9 @@ public abstract class SimpleNPC implements NPC, PacketHandler {
             case MAINHAND:
                 this.inHand = item;
                 break;
+            case OFFHAND:
+                this.offHand = item;
+                break;
             default:
                 throw new IllegalArgumentException("Entered an invalid inventory slot");
         }