Sudden Death

What you are paying for here is uninterruptibility. Black has always had cheap, efficient creature removal, and -4/-4 on an instant is a perfectly respectable rate that kills most things short of a fattened threat. But the rate is not why this card exists. Split second seals the spell behind a window nothing can pry open: your opponent cannot sacrifice the creature to a non-mana ability in response, cannot pump it out of range, cannot flash in a blocker, cannot crack a fetchland to shuffle away a tutored answer. Every reactive line that runs through the stack is locked shut. That makes it a precise tool against the things that normally lean on instant-speed protection: the regenerator who would rather pay to keep their threat, the opponent holding a counterspell over their combo piece, the deck that wants to respond to your removal with a sacrifice trigger. It is worth being precise about what split second does and does not buy, because the answer is narrower than it looks: it cannot stop triggered abilities, so a persist or undying creature still returns after Sudden Death resolves and leaves the stack, and it cannot stop special actions, so a face-down creature can still be turned face up while the spell waits. The cost of the guarantee is that it binds you too: with the spell resolving, you cannot respond either, so it is removal you commit to fully rather than hold up flexibly. The -4/-4 is ordinary; the inability to be answered is not.