Pay No Heed

Most damage prevention from this era forced you to commit to a single attacker or a single bolt; this one names a source and then walls off everything that source does until the turn ends. Point it at the biggest creature in a swing and the whole creature's combat output evaporates, blocks and trample math included. Point it at a burn spell on the stack (the spell itself is the source, so the prevention shield sticks to it as it resolves and after) and you blank a turn of that source's reach. The one-mana, instant-speed rate is generous for a hard "no" that scales with how much that source was about to do, which is the quiet appeal of source-targeting prevention over the per-damage and per-creature fog effects white usually got: against a single large threat it is total, against a board of small ones it does little. That asymmetry is the design's honesty. It rewards reading the board for the one source carrying the turn rather than buying you blanket safety, and it is at its sharpest answering a creature whose entire game plan is one oversized attack.