Theres also random needles if the dripping gore in the brain hole isnt enough for you
Asinine Antics puts the curse effect on all opposing creatures, reducing them to 1/1 stats, making for an easy board wipe
It's not very hard to break it, of course -- just add in Phyrexian Arena and/or Pitiless Plunderer and that pesky mana requirement is resolved -- but at least Kels completely broken all by itself like Korvold, Fae-Cursed King
Yep, more expensive, not less
As long as Knife and Death is on the stack, players can't cast spells, activate abilities, or move their permanents
by restricting the mechanic of playing cards during the opponent's turn to the attack phase, it should be fine to implement in TCGO (you don't need to have players pass priority during their turn, you only need to ask Player B if they want to play something when Player A uses an attack, once per turn)