Yes, they will end up together and Poliana will become the Empress. She will also give birth to a son.
Continuing on this timeline:
Rebecca gets pregnant but dies during childbirth. She gave birth to a daughter which saddened most people in the kingdom because an heir to the throne wasn't born. Poor princess. (She will be named Luminae and Poliana just adores her. She visits the princess regularly and enjoys hugging her.)
After Rebecca's death, Poliana mourns so much. She wanted to mourn with the Emperor however Lucius did not want Pol to see him so vulnerable. He was sad that the mother of his child died. He reminisced that his own mother died due to being weak and wasn't able to give him a sibling too. He thinks that he is lucky in other aspects in life, but unlucky with women.
Lucius drove Pol away and that made Pol realize that she was lonely and had no one to talk to or confide in. She locked herself up in her house for a month thinking of it as a punishment. When Lucius drove her away, she thought that she overestimated her importance to him and the house arrest was a form of atoning for that. She eventually decides to get married because of loneliness. At that point, Frau was the only one who dared to propose to her so she told him that she would marry him.
She's preparing for the wedding and of course Lucius is distraught. He tried to propose to Poliana but seeing that she is happy with Frau, he stops himself and wished for her happiness.
But lo and behold the Donau and Hau brothers arrived and exposed the kind of shit Frau truly is. Frau has a son whom he wants to be adopted into the marquess' family. Thinking that Pol is barren (she's not anymore, her period returned), he wants his son to inherit the Winter House. Upon knowing this, the soldiers who fought with Poliana in the war and the Emperor were beyond enraged. They wanted to kill Frau.
They decided, however, that it was best for Pol to fix this herself. When Pol knew about it, he ordered Frau to castrate himself and ordered him to never appear in front of her again. Should he do so, he will die by her hands.
Ah I forgot to add, before this Frau plot-reveal happened, Lucius is already wifeless.
One died, the other got her marriage annulled, and the other divorced him. So, he is left with a daughter with no heir.
After the castrating scene, both Lucius and Pol get drunk in a hotel. They were basically out of their minds from drunkenness as Pol was sad because she's thinking why is it so hard for her to get married while most of her colleagues already have families of their own. :'(
Lucius comforted her and because they were both drunk, they ended up having sex (This scene was so funny, you should read the novel)
Pol was horrified because she thought that she took advantage of her Emperor. Lucius was horrified too because he could not remember a single thing about that wonderful night. He kept banging his head to remember. Hahahaha
Lo and behold, Pol got knocked up. Her uterus was functioning too well, according to her. She didn't know what to do so she hid in Sitrin until she gave birth.
Currently, the translated novel is on the part where Pol already gave birth to a baby boy (his name is Gerald aka "Dog Stew" ε=ε=(ノ≧∇≦)ノ )
Read the novel if you have the time, it has soo much more details and it's really interesting! But it's darker than the webtoon so considered yourself warned.
Annulled- her faction (family) was plotting treason against the crown along with incredible amounts of corrupted funds. She helped uncover this. This was a particularly great scene in the story. It shows that women could also stand up for themselves and could think for themselves. Her marriage was annulled but she was given a noble title and a job in the palace making her the 2nd woman to obtain nobility for herself.
Divorced- she discovered that her own faction was slowly poisoning the other concubines along with her. The 3 concubines were so close, they didn't want to hurt each other and they treated each other closely as friends. She felt so much guilt that she was the one who asked for the divorce. Also, she was the first one who knew that Lucius was in love with Pol among the concubines.
Chapter 261 is where they arrived in an inn after Frau castrates himself (Pol's revenge). They get drunk here. It's not really explicit. They're just mostly talking and Lucius confessed his love for Pol all night. However, she couldn't hear it because she was also dead drunk and spouting things like "Your Highness, you're the most beautiful man in the woooorrllld!!"
Ok I can't wait anymore...Spoile me !