Here is a suggested edit for Caliber Arc but I need gsimenas' premission to add it. Yui asisted the party on the quest to recover Excalibur from Castle Thrymheim. Before the quest Yui and Liz explained to Klein about Magic, Skills, and the Elements (which he did not learn about since he did not read the instruction manual in the update). Before they went into Castle Thrymheim Yui warned them if they failed to get to Excalibur in time the quest would trigger the Ragnarok contingency and the cardinal system would destroy the game like what happened at the end of the Aincrad Arc in SAO. Yui also warned Klein not to free Freyja and thought that is was a mob trap. In reality this was Thor a quest NPC which they had to take with them to defeat the Frost Giants and retreive the rare item Thor's Hammer which in turn transformed into the rare item Lightning Hammer Mjolnir on compleation of the quest. Klein gave the hammer to Liz and it is her main weapon. Yui then told them to hurry and grab the sword and the party managed to reach the basement and Kirito pulled the sword out in time healing the World Tree and restored Jotunheimr to it's former glory.