You offer him the throne of Lametania if he swears loyalty to you and becomes your vassal.
6*
He thinks for a while, and then grins.
"Fine. As the third of Salus' sons, I was always pushed into the margin by my father and my brothers, but not anymore! I will show them that bullying me all my life was a gross mistake on their part!" He shakes hands with you to seal the deal.
Next morning, three-quarters of Alac's army marches out of the Asortum Castle; your officers quietly praise that you struck a deal with Alac, as it turns out that he had about two thousand men at the castle. Even after the castle would be ruined by Blackheart barrages, fighting or just displacing such large garrison would be hella difficult to accomplish. The remaining five hundred men stay at Asortum to repair the damage and man its walls.
Prince Alac also sends the envoy to the border keep ways in the north, as it is garrisoned by his followers as well.
Prince Alac himself provides you valuable information on the Republican mercenaries, who have been recently bought by Salus to provide defense of the capital. Also, Queen Karkhuta has been invited to conquer Lametania by Prince Gothem, a disowned son of Salus; as far as he heard, Karkhuta had him killed for battlefield incompetence recently.
He also explains to you the deal between his father and the bow champion, Regius, who have slain The Pillar of Quiet Disturbance; apparently, Salus swore to reward Regius with anything in Salus household the hero might want if the monster is killed. After the deed was done, Regius, either smart or arrogant (or both) decided to take Princess Ostia as his prize. Salus spent several hours discussing the demand with his sons, and ultimately decided to go against their advice and give Regius the hand of Ostia in marriage, and you know how that went.
Next morning, your officers convene with you, with Prince Alac sitting near you. Lots of ideas are exchanged; your officers want to besiege the capital now that the road is open to it, but Prince Alac wants to get rid of Felonsine invaders first, as he isn't that keen to become a King only to get his country chomped to bits by Felonsines.
You finally decide that the priority goes to...
A) Forced eviction of Queen Karkhuta and her army outta Lametanian soil.
B) Swift conquest of Lametanian Capital to remove Salus from the equation.