This story was discovered by @HistoryWeird. Their post is here. The original source was a letter, dated November 30, 1876, cited in Alexandre Tarsaize's Katia: Wife before God, 1970.
In 1866, the Russian Tsar Alexander II (known as Alexander the Liberator) visited a finishing school. He had been friends with Yekaterina 'Katia" Dolgorukova's father and upon her father's death, Alexander paid for his daughters' education. However, he had sent Katia away when she was younger and now, at age 18, he . . . uh . . . noticed her. He was 47 at the time.
Of course, how you could NOT notice someone wearing an outfit this ridiculous? I like that his portrait is looking in her direction. He's thinking "Hmm. I like what I see. I shall now 'liberate' you from your underclothes, madam."
Then, as most good kings do, he installed the girl he was trying to get it on with as one of his wife's ladies-in-waiting. His wife was conveniently dying of tuberculosis at the time, so I guess this is just some salt on the wound? "Hope you're feeling better honey, and also, here is a new maid who will replace you. Now hurry along and die, because we have some sweet lovin' to legitimize." Actually, part of Alexander's desire to have a mistress was that his wife had given him eight children and now, at her doctors' urgings, stopped being intimate with him.
Katia didn't want to be his mistress at first–she wanted something with more permanence. So she didn't sleep with him until after his eldest son died and an assassination attempt was made on him. That eventually wrenched open the doors to her feelings and they consummated their relationship. That night, he vowed that she was his secret wife and he would marry her if he were ever widowed.
Apparently they made up for lost time, though, because what followed was ten years of insatiable copulation: "She used a secret passageway to enter Alexander's quarters for passionate and sometimes torrid bouts of "bingerle" (their code word for sex). Alexander sometimes sketched Katia in the nude after which, according to his letters, they 'clenched each other like hungry cats'".
The doctors thought that Alexander's constant sexing was having a detrimental effect on his health, so they asked them to remain celibate for 6 weeks. In one of her letters during this period, Katia wrote: “I feel so heavy but I am not grumbling because it is my fault. And I confess that I cannot be without your fountain, which I love so… After my six weeks are over I count on renewing my injections.”
He saw her as often as his duties allowed, and they wrote to each other several times a day, usually inane letters about how much they enjoyed giving each other The Lovin'. All the same, they maintained an incredible level of secrecy despite the 10-year relationship and resulting three children. I'm not sure precisely when his wife figured it out (she kept clinging to life), but many people at court knew right away and were not pleased.
Finally, his wife died. On her deathbed she, being the world's BEST SPORT, asked to meet his love-children and she blessed them. He married Katia very soon thereafter, and legitimized their kids. "A year later, on the day that Alexander was assassinated, Catherine pleaded with him not to go out because she had a premonition that something would happen to him. He quieted her objections by making love to her on a table in her rooms and leaving her behind. Within hours he was mortally wounded and was brought back to the palace, broken and bleeding."
She was shunned by the rest of the family and court, and used her sizable inheritance from Alexander to move to Paris and become a fashionable hostess, which she did for the next 40 years.