No, you cannot. Activating an ability follows the same process as casting a spell (602.2b).
"601.2c The player announces their choice of an appropriate object or player for each target the spell requires. [...]"
Spells and activated abilities require legal targets to be chosen after being placed on the stack. If those requirements are not met, the activation will be illegal and the game will return to the moment prior to said activation (721.1).
In this case, if you try to activate Ertai, the Corrupted with no spell on the stack to target, the activation will be illegal. So, unfortunately, you can't use his ability as a cheap sacrifice outlet whenever you want.
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TheVectornaut says... Accepted answer #1
No, you cannot. Activating an ability follows the same process as casting a spell (602.2b).
"601.2c The player announces their choice of an appropriate object or player for each target the spell requires. [...]"
Spells and activated abilities require legal targets to be chosen after being placed on the stack. If those requirements are not met, the activation will be illegal and the game will return to the moment prior to said activation (721.1).
In this case, if you try to activate Ertai, the Corrupted with no spell on the stack to target, the activation will be illegal. So, unfortunately, you can't use his ability as a cheap sacrifice outlet whenever you want.
December 9, 2018 8:52 a.m.