Other jealous demigods could act as antagonists. They might feel that Percy doesn't deserve the blessings and try to undermine him. They could spread rumors or even try to sabotage his quests to make him look bad in the eyes of the gods.
The other gods who are jealous of Percy's new connection with Hades could be antagonists. For example, Zeus might see Percy as a threat now that he has Hades' blessing.
Perhaps the blessings make Percy a target. Other demigods or villains might be jealous of his new status. He then has to deal with the envy and threats while also trying to figure out how to fully utilize the blessings in a positive way. For instance, the gods' blessings might come with some sort of side - effect that he has to overcome as well.
It could also be that the Olympian Council as a whole acts as the antagonist. They might have made decisions that Percy sees as a betrayal, such as sacrificing the safety of demigods for their own political gain. Each god on the council, with their own self - interests, might contribute to Percy's feeling of betrayal, making the council as a whole the main force he has to oppose.
The main antagonists would most likely be the gods themselves. Since the title mentions being hunted by the gods, they would be the ones sending various threats Percy's way. It could be Zeus, who is often quick to anger and punish those he thinks are a threat to his power.
Chiron, the centaur trainer at Camp Half - Blood, is often included. He has a vast knowledge of the gods and their ways, so he can guide Percy through understanding the nature of his blessings. He can also help Percy train to control any new powers that come with the blessings. Additionally, some fanfictions might include new or minor characters who are either the cause of the blessings or are affected by Percy's new - found status after being blessed by the gods.
One interesting plot could be Percy getting a new and extremely powerful ability blessed by a previously unknown god. This could lead to him having to face a new set of challenges and enemies that are specifically drawn to his new power.
Obviously, Hiccup would be one of the main characters. Since it's about him being blessed by the gods. Toothless, his dragon, would also be very important. They are a team and Toothless would be by his side through whatever the gods' blessing brings.
It could be an old enemy from Greek mythology that has been resurrected. For example, a powerful Titan who has a vendetta against the demigods, especially Percy and Annabeth.
The traditional gods who don't want to see Percy as king could be antagonists. For example, Zeus might see Percy as a threat to his power and try to stop him at every turn.
It could be some of the more vengeful gods, like Ares. If Percy has somehow angered him in the past, Ares would be more than happy to hunt him down. Also, the gods' enforcers, such as the minor deities who do their bidding, could be antagonists too. They would be the ones doing the legwork to find Percy while the major gods give orders from Olympus.