In an era where most nations have opted for smaller and more economical options for its surface fleet, Russia has seem to literally go against the tide. The Kirov class are enormous nuclear powered cruisers with weapons jammed in to every available space. When they entered service, they sent shockwaves through Western military observers. They were genuinely concerned about what these leviathans can do their fleets. Though most are out service as of currently, they won't be for long.
Credit: Camera Operator: CWO2 TONY ALLEYNE via https://commons.wikimedia.org/wiki/File:Nuclear_cruiser_Kirov.jpg
Launched in the 1980s, they were initially designed to handle submarines. Later its purpose was changed to the more traditional role of a surface combatant: attacking other ships. These were more than capable of doing so with their huge arsenal. Their primary armament was the anti-ship cruise missile, P-700 Granit. Like the cruisers that carried them, these were massive weapons to fear. These 33 ft long missile can carry a payload of over 1,500 lbs of explosive at Mach 2.5 out to over 300 mi. If that wasn't impressive enough, they also are equipped with some the most sophisticated guidance systems ever. Unlike most missiles, the Granit has two types to use in conjunction. There's an inertial guidance and active radar system. Even more unique to it is what happens when it is fired in a swarm. When multiple Granits are fired at once, one of the missiles will climb to higher altitudes and provide target designation for the others in the group. If the designation missile is shot down, another will take its place. Even scarier, the Kirov can carry 20 of these. They are even capable of being tipped with nuclear warheads.
Beside their Granits, they packed several other offensive weapons. For anti-ship and anti-submarine purposes, they are armed with cruise missiles capable of deploying torpedoes or depth charges. These include the Viyuga or Vodopad , of which, can also carry nuclear warheads. In addition the ships can also be equipped with regular torpedoes. For guns, they have 100 mm AK-100 dual purpose guns. These are designed for taking on both water craft and aircraft. Like most dual purpose guns, they are autocannons capable of high rates of fire.
For defensive armament, they too are some of the most sophisticated in the world. The most advance of these is a seaborne variant of the S-300. It is said to be the most capable air-defense systems in service. They are long range surface-to-air missile that can go out to 56 mi at Mach 4. Other abilities include being able to target multiple targets, discern from decoys and jammers, and engaging ballistic missiles. It is even theorized they can shoot down stealth aircraft with the right adjustments. For shorter range, they are equipped sea-based versions of the Tor and Osa missile systems. In conjunction, they have several 30 mm Close-In weapons systems: the AK-160 and AK-630.Credit: USN via https://commons.wikimedia.org/wiki/File:ARKR_Kalinin_flight_deck_with_Ka-25_and_Ka-27.jpg
The Kirov class also has some other unique features that add to its capabilities. Like most modern capital warships, they carry helicopters for anti-submarine missions. But unlike most, they can carry up to 5 of their Kamov Ka 27 Helix helicopters. Though the normal complement is three, that's still one more than their nearest western counterpart. The propulsion system is also something not found on other warships. Besides being nuclear powered, it even has a system comparable to a supercharger in a car. The reactors are coupled with oil-fired generator that superheat the steam from the reactors. This allows for even higher speeds.
Other than its size and immense arsenal, the development of the Kirov caused concerns for the U.S. Navy at a very pressing time. During the 1970s the U.S. Navy found themselves in a so called "cruiser gap". Basically because of how they defined cruisers, they found themselves severely lacking in true cruisers. When the Kirov was deployed, they needed a stop gap measure to counteract it. This was one of the defining factors in the reintroduction and refit of the Iowa class battleships.
As of today, only one of them is in service: Pyotr Velikiy. However, there is one that is about to reenter service. Admiral Nakhimov is currently being upgraded. Also Admiral Lazarev is in reserve. If necessary she could be refitted and put back in to service. Given what is happening in Syria along with the rising tensions between the U.S. and Russia, the presence of these battlecruisers is definitely a cause for concern.