It trades off the upfront value of Sigil which makes immediate 4/4 flying angels, for the ability to make bigger creatures long-term and five your entire team flying and vigilance
This is solid by itself, but what really puts this card over the top is the incidental proliferate attached to it
Since joining The List in Wilds of Eldraine, however, this price has taken a bit of a plummet
We really saw how much people loved Ultimate MastersAs we went out and talked to people and were working on designing this set, Ultimate Masters was as like, Hey, this was a great Masters set. All right, lets use that and take those learnings and make sure to keep all of the goodness of Ultimate Masters and then add on the fun theme of double. No Fetch Lands in Double Masters When Wizards revealed Secret Lair: Ultimate Edition earlier this year, which contained reprints of the five enemy fetch lands, they said that there will be another way to pick up some stylized versions of fetch lands later this year that will also be in your local game store. Double Masters, however, will not be that way
For a broader look at where the format's staples land, see The Most Valuable Modern Magic Cards
My son, Adam, by the way, is a huge Minecraft player, so the most excited I have ever seen him about any Magic news I have ever told him ever was when I revealed that Notch was making a Magic card