Glacier

This is a pure damage synergy through attack speed for the whole board. Three heroes at 1 gold let you reach (2) in the first rounds, while (4) with its sharp jump in attack speed comes together in the mid game and is felt immediately, especially on heroes that build mana from auto-attacks. (6) needs the expensive heroes and pushes the ramp to the maximum: every attack speeds the piece up even further. The synergy gives no survivability, so the front line has to be picked separately.