You battled king slime and got your armor but who is this ninja?
Slimes attack on sight so could they have something against humans? Found everywhere but never attack other colours, and blue Slimes may be important.
King slime is sort of a rebel, against worship of gods (Humans/Players) which explains why slimes don't charge straight to enemies. They fear king Slimes and must do there bidding.
Looping back, the king slime may have fought a human and been harmed even though he managed to defeat it, the ninja! King Slimes are family and may be avenging death?
That at is my theory, comment your thoughts!