Project 09852, K-329 Belgorod ("Белгород"). Nuclear-Powered Special Mission 'Auxiliary' Submarine (SSAN) and Nuclear-Powered Drone Hosting Submarine (unofficially 'SSDN')
Belgorod is one of the least understood submarines currently being built. It was launched in April 2019 and started sea trials on June 25 2021. It will be armed with the Poseidon strategic nuclear weapon, and also perform secretive seabed warfare missions. In this latter role it will be operated on behalf of the Main Directorate of Deep-Sea Research (GUGI). It is unclear which role will take precedence.
The special missions of Russia's seabed warfare submarines is shrouded in secrecy. It is often discussed in euphemism by the defense community. They are referred to by terms like “seabed warfare” and “underwater engineering.” In layperson’s terms this means laying sensor networks on the sea floor, and possibly wire taps on internet cables. Belgorod will act as a host submarine (aka mother ship) to smaller submarines engaged in these activities.
Russia is planning a new network of naval installations in the Arctic. Principally among these is the Harmony sensor network (loosely like the famous US Navy SOSUS system) on the sea floor.
The logical aim is to provide Russia with real-time tracking of the latest NATO submarines operating in northern waters. This addresses the NATO submarine advantage head-on and would allow persecution of prowling US-UK nuclear powered attack submarines. In doing so it should improve survivability for Russia's own nuclear armed strategic submarines.
The seabed elements will be laid and maintained by the Russian Navy's special mission midget submarines. The Russian term for these is "Autonomous nuclear deep-water stations" (avtonomnoy atomnoy glubokovodnoy stantsii), or AGS. This acronym has found its way into western terminology.
https://www.navalnews.com/naval-news/2021/06/russias-gigantic-submarine-belgorod-sails-for-the-first-time/