Tundra Fumarole

Four damage to a creature or planeswalker at three mana is a fair, unremarkable rate; the second line is the reason to read the card. Every snow mana spent to cast it comes back as colorless, and because a Snow-Covered Mountain produces red that is itself snow mana, a fully snow manabase can float the entire cost back to you: cast this for three snow, get three colorless returned, and you have paid nothing net for the kill spell. The clause that "you don't lose this mana as steps and phases end" is where the structural work happens. Floated mana normally empties at the boundary of each step and phase, so a refund that vanished immediately would be worthless; instead it survives to power a follow-up spell or activation in the same turn. That is the older idea this design revives: mana-neutral removal that pays for your next play, here wearing the snow subtheme rather than a ritual or an artifact. The snow dependency is what keeps it honest. Strip the snow away and the second line does nothing: you are left with a plain three-mana burn spell that cannot even go to the face. It becomes a genuine tempo engine only when a manabase feeds it snow, tying its ceiling directly to a commitment most piles will not make.