The Platelegs of Trials is a part of the Armour of Trials which is obtained from May's Quest Caravan by allocating 100 quest points to the combat track. It has three tiers available, requiring 25, 50 or 75 Defence - and will automatically adjust up to the highest level available to the wearer.
The set effect is that whenever the player dies in a quest boss, each component of the set will reduce 1% of damage from said boss per death, up to a total of 5% per piece. This will grant a maximum of 25% total damage reduction if every component of the set is used.