I resume Emeria's ability requires a target, and if the target somehow got exiled the ability fizzled? What confused me is the IF clause that might allow the controller to simply shift to another target?
Emeria uses the word target so you are correct about fizzling. I'm not sure why you think the If would allow you to change the target. The only way to change the target of a spell after you have already chosen one is if the spell specifically says to such as Wild Ricochet. The "intervening if" of the ability stops the ability from triggering if the condition isn't met and stops it from working if you no longer meet the criteria when it resolves.
Emeria will trigger as your upkeep begins if you control 7+ Plains.
As for all ordinary abilities, it will then spend some time on the Stack, waiting to resolve.
When time comes for it to resolve, it will make sure once more that you (still) control 7 Plains (this is what an 'intervening if' clause does; see rule 603.4).
Therefore, while Emeria's ability is on the Stack, an opponent could destroy some of your Plains, in order to make its effect fail at resolution.
As for its target, you are correct to assume that the ability 'fizzles' if the target creature card gets removed from the grave.
Nothing allows you to change the target you've previously chosen.
I resume Emeria's ability requires a target, and if the target somehow got exiled the ability fizzled? What confused me is the IF clause that might allow the controller to simply shift to another target?
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As for all ordinary abilities, it will then spend some time on the Stack, waiting to resolve.
When time comes for it to resolve, it will make sure once more that you (still) control 7 Plains (this is what an 'intervening if' clause does; see rule 603.4).
Therefore, while Emeria's ability is on the Stack, an opponent could destroy some of your Plains, in order to make its effect fail at resolution.
As for its target, you are correct to assume that the ability 'fizzles' if the target creature card gets removed from the grave.
Nothing allows you to change the target you've previously chosen.
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules