Aether Spike

Force Spike scaled to a resource, and the resource is the wrinkle. The classic soft counter taxes for a flat amount; the target either pays a fixed toll or the spell dies. This one manufactures its own tax on resolution: two energy immediately, then a choice of how much to spend, each point demanding a matching generic mana from the opponent. That turns a fixed tax into a dial. Held with an empty energy reserve, it is a modest speed bump. Sitting on a stockpile from other energy payoffs, it becomes a hard counter dressed as a soft one, capable of demanding more mana than any reasonable player can float. The design trick is that the counter's strength lives outside the card itself, in whatever energy engine you have built around it, which is a rare thing for a two-mana instant: most permission is priced entirely on its own text. The floor is real (you get the two energy, though you choose how much to spend), and the ceiling is only bounded by how much energy you have banked. That places it in a lineage of tempo permission that trades reliability for cheapness, but it is the first of that lineage to let a wider board state set the terms of the trade rather than reading them off the card.