Sanguine Bond

Half of a combo, and the inert half at that: this enchantment does nothing until life flows your way, at which point it converts every point of gain into an opponent's loss. On its own it asks you to build a lifegain engine first and reach for the drain second, so it has always traveled best alongside its mirror image, Exquisite Blood, which gains you life whenever an opponent loses it. Stitch the two together and any single point of life movement loops until someone hits zero, a famous two-card kill that put both enchantments on every casual ban list and every Commander player's radar. The asymmetry of the trigger is the whole strategic point: it cares about the act of gaining, not the source, so a Soul Warden ping, a Whip of Erebos swing, a gainland's incidental tick all become reach you never had to spell out. That makes it a payoff that taxes nothing in your existing plan; you are not bending the deck around it so much as cashing in the life totals you were already padding. The cost of that flexibility is patience: the life it drains is loss, not damage, so it cannot be prevented, but it points at a single opponent and waits for you to do the gaining. A slower clock than burn, with no ceiling once the lifegain is genuinely flowing.