Pale Moon sits upright, rips off its shirt and lets out a whoop.
Legacy or Vintage sideboard, mana-denial shenanigans? It seems to jive with wastes colorless mana from Oath of the Gatewatch, so this seems likely to be the impetus for the spike. Clearly some power here, clearly a bit overlooked, not quite sure what the specific use is. Similar in effect to Infernal Darkness, legal in the same formats. Meta-game balancers like this are prone to spikes.
Vendors still have these for well under $1.00 as of publication (and in quantity!), whereas on tcgplayer.com they're cruising well-over $3.00 apiece. I left ya'll some foils; I ate my fill.