Instant
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If the spell mastery ability applies and a creature affected by Send to Sleep changes controllers before its old controller’s next untap step, Send to Sleep will prevent it from becoming untapped during its new controller’s next untap step.
Send to Sleep can target tapped creatures. If a targeted creature is already tapped when the spell resolves (and the spell mastery ability applies), that creature remains tapped and doesn’t untap during its controller’s next untap step.
If you chose two targets and one is an illegal target when Send to Sleep resolves, that creature won’t become tapped and it won’t be stopped from untapping during its controller’s next untap step (if the spell mastery ability applies). It won’t be affected by Send to Sleep in any way.
Check to see if there are two or more instant and/or sorcery cards in your graveyard as the spell resolves to determine whether the spell mastery ability applies. The spell itself won’t count because it’s still on the stack as you make this check.