It has been common practice that medieval nobles would have illegitimate children. Chief examples include Henry I, who had 29 children (5 of which were legitimate) and Henry VIII, who had a large number of illegitimate children. To make my fantasy series as close to medieval standards as possible, I am thinking of my protagonist producing several illegitimate children during his conquest. However, he will not sire these children on a whim like most nobles. Instead, he will sire them as part of a ransom demand from his enemies. There have been cases of monarch granting their illegitimate children titles and lands such as when Henry VIII gave one of his illegitimate sons a dukedom. Cases like these would result in the creation of cadet branches from the main bloodline. A prime example of medieval cadet branches were the House of York and the House of Lancaster, which were offshoot branches from the main Plantagenet royal family during the War of the Roses. This will ensure that if anything happens to the main bloodline that someone will be able to inherit the throne.