He had the task of tending the Trollbane family's rarer artifacts — back when they were merely kept in a locked room and not in the crypt. When the family manor was hit with a surprise bandit raid while much of the house guard was out fighting the Horde, Yernim hid the artifacts in the kitchen, filling the priceless chalice of opal voices with gravy, tossing the ancient sword Trol'kalar into the pile of pig offal and putting the cloak of elemental protection on himself and jumping into the massive kitchen fire to hide.
Prince Liam, grandfather of Galen Trollbane, was so impressed with his servant's ingenuity that he appointed him Caretaker of Artifacts with a staff of two, and commanded the young servant to come up with a better plan to protect the artifacts. Yernim had stonemasons carve another room out of the rock of the Trollbane Family Crypt for the artifacts, wisely allowing room for more artifacts as time went on. He took his underlings and taught them how to care for the artifacts properly, what each of them could do in case they needed to know for their own safety or if the artifacts were in danger (like he knew to hide in the fire with the cloak), and preservation methods.
When nobles of Lordaeron, Khaz Modan and Azeroth would visit Prince Liam, he would proudly tell the story of how Yernim had protected the family heirlooms while the guard was away. The other nobles were so much impressed with the system he had built that they asked to borrow Yernim for designing similar setups for their artifacts. Yernim, perhaps showing himself wiser than these great lords and ladies, mentioned they probably wouldn't want him knowing all of their secrets. He instructed them, with Prince Liam's approval, to send five trusted servants to him for training. Thus the continentwide training of the Caretakers began.