Lightning Surge

Threshold turns this from a serviceable burn spell into a finisher, and the upgrade is steeper than most of the mechanic's other beneficiaries got. The four damage to any target becomes six, and that six can't be prevented: the targeting stays identical (creature, planeswalker, battle, or player), since a planeswalker is a permanent, so "that permanent or player" covers the same legal range. What changes is the payoff, a clean, unstoppable burst that walks past fogs and damage-prevention shields. Then Flashback hands you the spell a second time from the graveyard. There's friction in the loop worth naming: flashback exiles the card on resolution, and every other spell you flash back or otherwise exile is one fewer card counting toward Threshold, so a deck leaning on this for its big number has to keep refilling the yard faster than its own engines empty it. The cost of all this is the rate. Five mana for the front half and seven for the flashback both arrived in an era when red's premium removal was charging far less, so this was always a deck-defining commitment rather than an efficient inclusion. What it offers in exchange is reach: two bursts of damage from a single card, a different currency than tempo, and one that pays off in a deck already churning through its graveyard for reasons of its own rather than treating Threshold as a tax to pay.