The Nuclear Silo is an ultimate introduced in the 13.0.0 update.
It appears to be a futuristic black, silver and white opened silo with four red lights and a large nuclear missile on it.
It would allow the fort to inflict a nuclear strike on the battlefield periodically and destroy all enemies.
At this moment, ultimates like this are not in the game. It is expected that ultimates will be added to the game at some point due to the fact that the player is able to browse them in the clan fort section, and also in the clan storage, which is quite misleading. At the moment, it is unsure if an ultimate will be an ability used once per game, or if it will be constant.