Why two different mana costs?
I changed the mana cost because I changed the type from Sorcery to Instant. This card could also cost UUUX as an Instant, or UBX as a Sorcery. (U is what we use for blue because B is black.)
Changing the type from Instant to Sorcery generally makes a card much more powerful, especially for blue. If it were a Sorcery, you would have to use it during you main phase and devote a turn to casting it. On the other hand, if it's an Instant, you can wait until the end of your opponent's turn to cast it giving you the opportunity to do other things with your mana if need be (countering spells, etc.).