It was a secret unit that required a nuclear reactor to build in Call of War 1.0, along with nuclear carriers and battleships. They have been removed as of CoW 1.5, but the achievements remain.
The Mediterranean and Tournament Island maps, for which achievements still exist, no longer are playable either.
I suspect the reason they keep these achievements is that some people still pocket the gold from having built these, so removing them would cause those people to lose their gold. But they could reimburse.
