Each rad counter represents a life point and a milled non-land card, plus however many milled land cards are in between
Tournament legality fuels steady demand among competitive players
If you combine Heliod and Spike Feeder with Conclave Mentor, you can make one creature infinitely large, since you get an extra counter with each Heliod trigger
I can see it being a four-of in some Black aggro decks, essentially pre-sideboarded against sweepers, or an insurance policy post-trading
Dualcaster Mage Dualcaster Mage shines in infinite combo setups, particularly when paired with cards like Twinflame or Heat Shimmer
The restriction of only one target per turn is somewhat diminished by the fact that this card enables you to play green, and cards like Eternal Witness or Regrowth are very welcome recursion enablers in a deck that's already grindy by itself