I wonder if I am able to cast Killing Glare if there are no powerless creatures on the board?
For example to Extort after casting.
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No, you have to pick a legal target (a creature with power 0 or less) when you cast the spell, as instructed by the third step of casting a spell.
601.2c The player announces his or her choice of an appropriate player, object, or zone for each target the spell requires. [...]
If you can't, any steps of casting the spell already taken are undone.
601.2. [...] If, at any point during the casting of a spell, a player is unable to comply with any of the steps listed below, the casting of the spell is illegal; the game returns to the moment before that spell started to be cast [...]