Kaervek the Merciless

Whenever an opponent casts a spell, Kaervek the Merciless deals damage equal to that spell's mana value to any target.

Heartless Hidetsugu

{T}: Heartless Hidetsugu deals damage to each player equal to half that player's life total, rounded down.

Whenever an opponent casts a spell, that player loses 5 life unless they discard a card.

At the beginning of each end step, each opponent loses life equal to the life they lost this turn. (Damage causes loss of life.)