Smoke Bomb

Board-wide shroud is a defensive effect dressed up as a symmetrical stall, and the wrinkle is who actually benefits from it. While it sits on the battlefield, nobody can target any creature: no removal, no combat tricks, no pump, no protective auras from anyone, including you. That reads as a lock until you notice the artifact has an expiration date built in. It sacrifices itself on your upkeep, and the sacrifice hands you an evasion trigger, so the card is designed to be shed the moment it stops earning its keep. The sequencing is the whole trick. Flash it in at the end of an opponent's turn to blank whatever targeted removal they were holding for your attacker, ride the shroud through your untap, then trade the artifact for an unblockable strike on your own turn. It is a one-turn insurance policy that cashes out into offense rather than lingering as a permanent that has to be answered. The tension is that untargetable creatures cut both ways: you cannot protect them with spells while the shroud is up either, so the card wants a threat that is already resolved and just needs to survive the swing back. It rewards a plan built around a single creature you intend to push through, not a board you want to keep interacting with, which is a narrower brief than "all creatures have shroud" first suggests.