If the target creature is an illegal target as Hydaelyn, the Mothercrystal's last ability tries to resolve, it won't resolve and none of its effects will happen
Fortunately, Sphinx/Shadow of the Second Sun can help, untapping your lands and transforming Emet-Selch before your second main phase
With great card draw and aggro both in the command zone, and enough little creatures to get said commander out early with Convoke, it's fairly easy to start some life totals rapidly declining while also drawing significant portions of your deck to keep the whole thing going through a possible board wipe
Speaking of being rewarded for doing what you were already going to do, plenty of white decks will be enthused to see a curve topper that keeps the cards coming
The Beginning Phase is divided into three steps: Ready, Set, and Draw