Purge

Two mana, and it kills almost nothing: only artifact creatures and the whole of one enemy color, and not a single creature outside those two lines. That is the entire point. An unconditional two-mana creature kill would warp any environment it touched, so white's access to destruction this cheap gets fenced off behind a targeting wall, and the wall does exactly half the work, since roughly half the creatures the spell could ever face are simply immune to it. The "can't be regenerated" rider matters more than its placement suggests: in the era this kind of design came up, black fielded regenerators that survived ordinary removal and walked back into combat, so stapling the clause on guarantees the spell finishes the job against the things it was aimed at rather than getting blanked at the last step. This is color-hosing as a deliberate lever: the rate is real, even punishing, but it is payable only against a known target, so the card never becomes a default answer the way an open-ended kill spell would. You spend almost nothing, but you only get to spend it when the board hands you the right thing to destroy. The lesson lives in the restriction itself, which is the cost that licenses the aggression on the front side.