The way in which you realise that the shadow has turned against you is via receiving the debuff
Devastating Filth Curse.
This is a debuff that lasts for TEN years and prevents you from receiving any money drops from enemies, not very pleasant.
Having no money drop is not very nice at all, you likely want mobs to give you your hard-earned reward when you kill them.
This problem can be rectified by visiting your old friend The Necromancer. On human side this is at Vassals' Tombs.
He proceeds to tell you he cannot help you, but directs you to Lady Dawn at Lumirya Waterfall, as she is the expert on delinquent shadows.
She asks you to talk to the shadow, you find it only speaks Russian. *audible gasp* The problem is worse than you could have ever imagined!
Lady Dawn also clearly knows this is a big problem, so she gives you a special dagger that can be used to whip a rebellious shadow into shape. This puts you in a fight with your shadow who uses various potions and superblows to try and fight you off. Of course you will win the battle, right?
Luckily your beatings have also managed to teach your shadow English. Hooray! After some convincing from the shadow you are now happy with your loyal shadow buddy. You return the dagger to Lady Dawn, she has many more players' shadows to rescue from their Russian delusions and greedy coin-stealing antics. The shadow also gives you a fancy looking
Dancing Shadows Mirror
Your shadow tells you to use this in case he catches the Devastating Filth Curse again, it can be used to cure your shadow without having to go pester the fine Lady again. It's also great for applying make-up with.
Voila! Your new, high maintenance, expensive friend is ready to roll out and fight your foes for you, for a price of course
.