Briarblade Adept 

Creature — Elf Assassin
Whenever this creature attacks, target creature an opponent controls gets -1/-1 until end of turn.
Encore {3}{B} ({3}{B}, Exile this card from your graveyard: For each opponent, create a token copy that attacks that opponent this turn if able. They gain haste. Sacrifice them at the beginning of the next end step. Activate only as a sorcery.)
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Cards that do something similar expected a card that isn't here?
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Escaped Experiment 97%
$0.11#21633
"Whenever this creature attacks, target creature an opponent controls gets -X/-0 until end of turn, where X is the number of artifacts you control."
Vedalken Mesmerist 93%
$0.02#29049
"Whenever this creature attacks, target creature an opponent controls gets -2/-0 until end of turn."
Binding Geist // Spectral Binding 93%
$0.06#25506
"Whenever this creature attacks, target creature an opponent controls gets -2/-0 until end of turn."
Spring Splasher 91%
$0.18#17734
"Whenever this creature attacks, target creature defending player controls gets -3/-0 until end of turn."