It reduces only the generic mana component of that spell's cost
If you go wide in rakdos, I could see this acting as a surprise finisher that's still a very decent value piece before you're set up
While Mox Amber does require you to have a legendary creature or planeswalker in play to add mana, given the nature of Commander and the huge number of legendary creatures and planeswalkers Magic has been printing lately, it is very easy to activate
Should you not have other ways to manipulate the library it is far harder to milk any advantage out of Divining Top
Dark Confidant, or 'Bob' as it's often called by the Magic community, was once considered among the best MTG cards of all time
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