Majesty "M"

M is the father of K and a mysterious yet influential figure in Haven City. While there is much talk about him, for the most part no one quite knows which rumors are simply rumors and which hide a bit of truth in them. What is known is that he is high nobility and holds a very large, well-off domain. So well off that it boasts the highest number of nobles in the region controlled by the Haven City Government, many of whom M has bedded. As a result, M has 25 children each bearing a name that starts with a different letter of the Common alphabet, though most people are unaware of the exact number of children.
These children were all born around the same time and each named by M, though they were never told their true names. Instead, each is referred to by the first letter of their name. Upon coming of age, the children were made aware that the closer to the letter M their name was in the alphabet, the greater their potential (courtesy of an oracle in M's employ). They were told they can only earn the knowledge of their true names by going to the Underdark and surviving a trial there. Each of these children has felt a pull towards the Underdark ever since they entered adulthood, some metaphysical force drawing them towards whatever fate M set in motion for them.
He also seems to have some level of connection to the fey courts, as both the River King and Red have commented upon M and seem to hold him in an unfavorable light.

Welcome to Haven City Arc

M was briefly mentioned by his son K on several occasions, exclusively in a negative light. K seemed to possess a deep hatred for his father, as well as an intention of killing his old man at the first opportunity.

Journey Into the Underdark Arc

K mentioned his bastard of a father several times throughout the Delvers of Secrets adventures in the Underdark, but it wasn't until they met Mother Moth that the topic was addressed in ernest. K had seen several moths in their time down here; each time his vision turned red and his mind filled with rage, an impulse urging him to kill the moths. Despite this, one such moth eventually led them to the lair of Mother Moth herself. She had hidden herself away beyond an iron mine in the Shadowfell, impossible for a fey like M to approach. This was the reason he had father K and K's many siblings, to eliminate her as his rival.
Mother Moth informed K that the hatred he felt for her was not his own but M's, tranmistted through K via a Seething Sight curse. She also stated that M had tricked not her but K's mother (who had been a warlock under Mother Moth's patronship) in order to steal a sizeable portion of Mother Moth's power. She advised K to set his negative emotions aside, stating that M was a member of the Unseelie Court and could use such emotions against him. She instead encouraged him to focus on his love for his fiancée, Sonya Snowborne, and use those feelings of love as an antithesis for M's power. But she also warned K that M would inevitably learn of Sonya and try to use her against him.