User blog comment:TheCarrotSaysYumYum/FT0 - Game of Truth, Review./@comment-24066070-20140817132729/@comment-24066070-20140818064204

It is likely very rare, otherwise Ivan would've looked for another one in order to force Laxus to be honest with him during the GMG.

Even Tartarus didn't have one when they were interrogating Erza or perhaps they just preferred torturing their victim.

Perhaps only Treasure Hunting Guilds know where to find these treasures or how to make them.

About the Zeira - Sayla connection, that would be cool. Perhaps the reason Sayla likes reading and constantly throws books at her opponents is because she used to be a "librarian" on Tenroujima along with Mavis.