ICBM is one of four satellites introduced in CI4. It's three ballistic missiles from the Cold War.
Although quite ineffectual against single targets, its exceptionally large blast radius can damage dozens of enemies at one time. ICBM rockets unfortunately travel very slowly, so their strategic use is of prime importance.
With it, the tip is that best probablity to use ICBM is large waves, with many enemies, because with even only one missile from it, dozens of enemies will be killed fast.
The enemy hit with it will be dealt 5000 damage, and other enemies caught in the blast will take up to 1000 damage, depending on how close they are to the blast.