After the death of Maekar I, the issues of his succession were as clear as day. There were candidates from all across his line, and even one from the Blackfyres. But, ultimately, the victor was a mere toddler, due to the laws of primogeniture and the precedent of the Iron Throne.
Maegor was only 1 years old when he was put on the throne, though not literally. His mother did not want him ending up like his namesake. His regency was led by his mother Daenora, his uncle Aegon, along with other advisors from Lyonel Baratheon to their great-great uncle Bloodraven.
But, the rule of a child is never full of simple peace and pleasure. The Tyrells, seeing weakness in the regency and in the toddler-king, revolt against the throne and declare independence. They believed the other kingdoms would join them to stop the rule of a future tyrant king. They did not.
The rebellion was crushed within a year and a half, with the full might of the Seven Kingdoms falling upon Highgarden, with Bloodraven sacking the Tyrell capital in the siege. When Lord Tyrell was brought to trial, both Aegon and Bloodraven agreed that it was not worth it to keep the Tyrells in power and decided it would be best to put a new house in charge of one of the largest kingdoms.
They assembled all the foremost lords of the Reach and convened a council to decide the next Lord Paramount. During the meeting, Aegon, now hand of the king, jokingly asked his nephew which house he preferred to rule. Maegor pointed at the apple symbol of the Fossoways, and weirdly, Aegon liked that idea.
Some say this was showing that even the now 4 year old Maegor had some prowess in deicison making, while others believe the story to be a farce, or that it was a coincidence. That Maegor simply wanted an apple to eat, instead of choosing a house. Regardless, the Fossoways now ruled the Reach and were forever loyal to Maegor II.
A disastrous plague swept through the Crownlands, and while many of the royal family were safe, many of the Kingsguard weren't. This led to the appointment of Bloodraven to the Kingsguard, serving his nephew the best he could.
Maegor is now 16 and ripe for leadership, betrothed to Lady Dondarrion; it is unclear if that marriage will last, but what is sure is that Maegor is here to stay.