You can find the RW and BG ones first printed in Edge of Eternities Commander Precons, and the UG ones printed in the Lorywn Eclipsed Commander Precons
For example, an X mana symbol does not usually add anything to a cards mana value, except in specific situations
Since Dimir Excruciator is the hot new thing, the current high demand is unlikely to die out overnight
When creatures are cheap, decks start packing cheaper spells to deal with them, and thats when Chalice of the Void is at its strongest
If you have scry cards and abilities that you can activate on your opponents turns, it gets even better
Pack a Retribution of the Ancients, Together Forever, or Blade of the Bloodchief to turn it up to eleven