Russia Prepares to Deploy Highly Advanced Silent Submarine - NewsBreak

We don't know what weaponry is onboard their diesel boats, but... Ukraine has nothing to deter a loitering submarine off its coast from lobbing cruise missiles inland.
Diesel boats are very silent and dangerous to any shipping going on as well. I don't think this will be a total game changer, but it does change the calculus on where you put air/missile defenses.

Lt Col John (Jack) Christensen
..."Russia's Black Sea fleet, which can operate close to Ukraine's coastline, also has the Project 636.3 submarines, and is "actively involved in Russia's special military operation in Ukraine," TASS reported in June last year, referring to the war using the Kremlin-approved term. They can fire cruise missiles, such as the Kalibr missiles frequently used by Russia to attack Ukrainian targets.

These submarines have a length of 240 feet, and can travel at a speed of up to 20 knots, Russian state media reports. The attack subs are manned by a 52-person crew, and can reportedly stay out at sea for 45 days.

As of the start of 2023, Russia had 10 Project 636.3 submarines, according to the 2023 Military Balance, produced by the International Institute for Strategic Studies think tank.

The deployment of the Ufa diesel-electric submarine is important "because the Asia-Pacific region has become an arena of confrontation between great powers, primarily China and the United States," the Izvestia report said.

The Pacific Fleet, outfitted with these stealth diesel-electric submarines, therefore "needs to be maintained at a certain level," the state media outlet added."