As with most hero stories, the tales of his bravery were greatly exaggerated over time. He later admits that he himself had started the exaggeration. While his original stories had him luckily defeating the monsters, he actually stumbled across them already defeated, and just ensured they were gone before extracting a talisman.
As a ruler, he was summoned to fight a pillager that was called dragon. This would be actual combat, so he could not rely on luck. However, he could use the talismen that he procured and would defeat him in a David vs. Goliath contest. This probably contributed to his reputation as a "dragon slayer". It was only later (in a time not covered by the book) that he would actually fight dragons.
The takeaway seems to be that nothing ventured, nothing gained. There were plenty of other legitimate warriors. They could have gone out and gathered the rewards he did. However, none of them were willing to go. Being willing lead him to the initial rewards. However, that was not enough. He would have failed the final battle if he had not used the experience from what he previously accomplished.
The conclusion to Dragonwatch brings everything to a conclusion and ties up many of the loose ends. The characters complete many great journeys. While Kendra's journey was month important to Fablehaven, Seth's journey is key to Dragonwatch. He undergoes the great quest and completes the hero's journey. The journey includes many fantasy and religious themes with a twist. Seth must journey to the darkest of dark realms and suffer immensely before he can finally be submerged in the water that heals. An innocent being would serve as the guide to bring him there. An immortal being must willingly end his life to help restore that which was broken. 
The Titan Games are a popular event lead by Humbuggle, a powerful troll. A giant stadium brings people from all over. There they watch gladiators battle. It is all part of of a long scheme of games to allow the victor to take a precious stone which grants immense power. Seth goes there in an attempt to gain his memories back. In the process he runs into separated memories (that he helps cristen as "Reggie"), a satyr professor and many other beings. 
Seth joins the darkside in this third Dragonwatch book. A "dark unicorn" took him away and has him continue to run errands to support the undead and other dark creatures. Seth has lost his memories and is not sure who is telling the truth. He feels it is bad to have people imprisoned, so helps free the creatures and run other errands for the dark.
The dragons have declared war. They want full control and don't want to be controlled by humans. They are after a special stone that will all them this power. The humans don't want them to have it. They are seeking it also. However, the troll guarding it has other things in mind. Those that do something bad on their own are forever cursed by his game, but those that had something bad happen out of their control are restored. We also get to see many more fantastical creatures, such as talking animals and a living forest. The cousins gain the ability to see the magical creatures, but cause some havoc on their own. A "memory loss" door causes interesting things to happen. Like a typical middle book, it opens more questions than it closes.
The Dragonwatch series kicks off where Fablehaven ended. Seth goes on a mission to return the sword, and discovers that he will have a future with Dragons. Back at Fablehaven, the kids are with both sets of grandparents. Things get interesting when their cousins visit. They don't believe in the fairy stuff. However, this eventually becomes a benefit when hunting for gold or playing a "super VR capture the flag".