You might take away their fourth land or their preferred three drop but that won't actually do anything for two or three turns which greatly reduces the value of Blackmail being one mana

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Activate only once each turn
Keep him on the board long enough, and Vivi becomes a powerful attacker in his own right, but his real value lies in the win conditions you can create with the killer combination of free mana and pinging
TheChirurgeon: It's hard for seven-mana walkers to deliver enough value to see play, but Chandra manages to avoid that, both because she can drop 8 damage and get some easy 3-for-1s the turn she hits the battlefield, and because her +1 ability is capable of creating immediate infinite combos with things like Felidar Guardian, The Peregrine Dynamo, and Combat Celebrant