The Planar Die is unique to the Magic Planechase expansion sets. This die is used during gameplay to move players between planes by revealing a new Planar card and triggering the abilities of that card.
You can visit wizards.com for more information on the Planechase expansion sets.
There are 96 pages, 90 for games and 6 for whatever you want. The pocket in the back is great for storing loose singles and the fold up cover design allows for it to unobtrusively fit on the table where it will never interfere with your game.