The Shield of Dismay is a shield that was only obtainable as a reward from the "A side of Justice!" quest that was obtainable and doable only on April 1, 2017 as a holiday event. Though there are still players that have the active quest in their log, there is currently no way to complete it.
This is the best shield in the game, and can go for millions. There is no upper price limit on this item.