Maybe a powerful and dark sorcerer who has the ability to manipulate the very fabric of reality. He sets up a situation where Percy is doomed. His goal could be to disrupt the balance between the gods and demigods by causing the tragic death of Percy, which would happen right in Poseidon's arms, creating chaos and despair.
It could be an ancient and powerful Titan that has risen again. This Titan might have a vendetta against Poseidon and targets Percy as a way to hurt him. Since Percy is Poseidon's son, his death in Poseidon's arms would be a great blow to the sea god.
One possible plot could be Percy sacrificing himself to save his friends while in Poseidon's arms. Maybe there's a great battle against an unbeatable foe, and Percy realizes the only way to victory is through his own death. He accepts his fate in Poseidon's comforting embrace.
Perhaps a powerful and corrupted demigod could be the main antagonist. This demigod might be jealous of Artemis' power and popularity among the other demigods. He devises a plan to harm her, and Percy, being loyal to Artemis, has to give his life to save her from this rogue demigod.
A powerful Titan who has escaped from Tartarus. This Titan blames Percy for his previous imprisonment and wants to make Artemis suffer as well for her role in keeping the Titans locked away. He uses Percy's death as a way to manipulate the situation and turn the demigods against Artemis, or he might try to steal Percy's powers that were left behind after his death for his own evil purposes.
It could be an old enemy from Percy's past life who has also found a way to be reincarnated. This enemy might hold a grudge against Percy for something that happened in their previous encounter and is now seeking revenge in this new life.
A powerful Titan who has a grudge against Poseidon could be a great antagonist. This Titan might be trying to undermine Poseidon's power by targeting his first - born, Percy.
A powerful Titan who sees Percy as a threat to his plans. Since Percy has the power of Zeus and Poseidon, he could potentially stop any chaos the Titan wants to create. The Titan might try to turn Percy's friends against him or find a way to strip him of his powers.
It could be a powerful Titan who has escaped from Tartarus and blames Artemis for something in the past. So he's out to get her, and Percy has to stop him at all costs, even if it means his death.
It could be an ancient and powerful god or goddess who has a grudge against Percy. Maybe they see Percy as a threat to their power or plans, so they plot his death. For example, a forgotten deity who wants to regain their former glory by getting rid of Percy, who is often seen as a hero in the Percy Jackson universe.
It could be a new, powerful Titan that was previously unknown. This Titan might have a vendetta against Percy specifically, seeing him as a major threat to their plans. Maybe they manage to kill Percy, but his rebirth sets in motion a series of events that will lead to their ultimate downfall.
It could be a new Titan that has been awakened from an ancient slumber. This Titan might be angry at the gods for imprisoning them long ago and wants revenge. Percy's death could be part of the Titan's initial plan to strike fear into the hearts of the gods and demigods, but Percy's rebirth becomes a major obstacle for the Titan.