This page of the Witcher 3 Wild Hunt guide contains a walkthrough for Cat School Gear Upgrade Diagrams quests taking place in Vegelbud Residence area. This is one of treasure hunt side quests - complete them to attain new diagrams.
Cat School Gear Upgrade Diagrams - Part 3
In a cave at the shore at the north of the location there's a golem (level 19). He protects the treasure - but the treasure is not the only thing that you should be interested. After defeating him you will be able to take cat school gear diagram - it's a sword diagram - lying in one of the chests in the corner of the cave.