'Doomsday’ Submarine Armed With Nuclear Torpedoes Delivers to Russian Navy
"Poseidon" (also known as "Skif" when in seafloor-launch mode) is a torpedo that can supposedly cruise the oceans or lay dormant on the seabed almost indefinitely.A Russian Navy submarine armed with a strategic nuclear torpedo the size of a school bus was delivered to the Kremlin this week, according to an announcement from the shipyard.
Project 09852 Belgorod is based on a Russian Oscar-class guided-cruise missile submarine that has been altered to accommodate six 80-foot Poseidon nuclear torpedoes that could be armed with a warhead of up to 100 megatons, USNI News previously reported.
The submarine was delivered to the Russian Navy in a ceremony in at the Northern Fleet’s headquarters in Severodvinsk on Friday.

Belgorod herself is fairly unremarkable, except for the ability to launch "Poseidon", and piggyback a deep-diving midget sub (possibly for tapping undersea cables).

Belgorod may also have the ability to launch torpedoes known as "Kanyon".

From Covert Shores:
"Kanyon" (Status-6) Strategic Weapon
The weapon is designed to strike coastal cities and strategic targets, e.g. New York, Boston, Los Angeles, San Francisco and bases like Groton CT and San Diego. The warhead is reported as a 100 megatons (which is credible) nuclear device with a ‘dirty’ shell (reportedly Cobalt, but likely Uranium) to maximize the radioactive fallout. The payload is similar to the warheads used in ICBMs (Inter-continental Ballistic Missile) but only one is carried on the torpedo. It could therefore be compared to a city being hit by a single MIRV (Multiple Independent Reentry Vehicle) except that it explodes under or on the surface of the water. The effect is likely to be much more localized than an air-burst, but with greater local contamination spread by a radioactive ‘rain’. The explosion itself may be some way out of the target city due to geography and obstacle defenses but a shoreline city like New York would wiped out by a single hit. Performance variables:
Depth: The stated running depth of 1,000m is credible and places it below current countermeasures.