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Q: With Warden of Evos Isle on the battlefield, can I bestow Celestial Archon for less colorless mana even though I am not casting it as a creature? —Jay
A: Matt Tabak, rules manager:
The short answer is no, Celestial Archon cast with bestow doesn't qualify for the discount.
For the slightly longer answer, we have to look at how spells are cast. You don't calculate how much a spell costs until pretty late in the process. The first thing you do is put the spell on the stack. Then you make choices relevant to the spell such as choosing modes (not very likely), choosing what cards you're splicing onto the spell (even less likely), and choosing what additional or alternative costs you'll be paying (bingo). It's here that we declare our intention to pay the bestow cost. This causes the spell to become an Aura spell with enchant creature. Then we choose a target for the spell. Finally, we calculate the total cost for the spell. At this point, Celestial Archon is not a creature spell, so the Warden of Evos Isle discount doesn't apply. There's a chance to activate mana abilities, and you pay the spell's costs. Voilà! Celestial Archon becomes cast.