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
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
You can see white dots and dirt in certain spots
Fortunately, Sphinx/Shadow of the Second Sun can help, untapping your lands and transforming Emet-Selch before your second main phase
Prepare yourself, take your cards, during OP-05 we will win our precious ticket for Japan to become the King of the Pirates
This first effect of Abyss Dweller is negligible, as its 500 ATK boost when it has a Water monster attached to it usually does not matter throughout a game