For two decades, Ramzan Kadyrov has been the Kremlin’s iron fist in Chechnya. In return for helping to brutally suppress an independence movement, he has been allowed to rule his region as a personal fief, crushing rivals and dissenters, as well as separatists.

But Mr. Kadyrov, 48, appears to be seriously ill, presenting President Vladimir V. Putin with a new challenge in a part of southern Russia where wars killed tens of thousands in the 1990s and 2000s. When the strongman leaves the scene, who can maintain the brutal control he has imposed over that part of the Caucasus?

Mr. Kadyrov’s own succession plan could be resting on his 17-year-old son, who got married and received congratulations from Mr. Putin over the weekend. But that would require circumventing Russian law requiring regional leaders to be at least 30 years old.

And there are other contenders, including a man who led an effort to round up and brutalize gay people and another who went off to fight for Russia in Ukraine.

Over the years, Mr. Kadyrov has become a unique figure in Mr. Putin’s autocracy, enjoying far more latitude and richer subsidies than other regional leaders. He commands what amounts to his own army. He has imposed strict Islamic rules in his mostly Muslim region that contravene Russian law, and he has conducted something of a foreign policy of his own, fostering ties with Gulf monarchies and taking positions that do not always align with the Kremlin’s.

Chechnya was the only region exempted from Russia’s conscription of about 300,000 men after its 2022 invasion of Ukraine. But the authorities did send some Chechens to the war as punishment, according to rights activists and local residents who spoke to The New York Times. And though Mr. Kadyrov committed some forces early in the war, it soon became clear they were not doing much fighting, earning themselves the nickname “TikTok soldiers.”