Pride was Theseus' fatal flaw.
He abandons Ariadne, the literal only reason he survived the labyrinth, because he feels he can do better or because he doesn't want to acknowledge that he couldn't defeat the minotaur and survive the labyrinth alone. He gets so caught up in his victory that he forgets to raise white flags so his (mortal) father knows he survived, and said father throws himself into the sea in despair and dies thinking his son is dead. Becoming King of Athens as a result of that, he becomes a tyrannical king because of his pride and because a toxic friend uses his pride to manipulate him. Where once he was brave and cared for other people, only attacking after first being attacked, after meeting Pirithous, he began doing dangerous and cruel things without thinking about the consequences of his actions. He becomes cruel and self-serving. He kidnaps a thirteen year old Helen of Sparta, intent on making her his wife (never mind the wife he already has) because she is a beautiful daughter of Zeus, the most beautiful girl on the Earth, who else would be worthy of her?, he thinks. He tries to help his friend kidnap Persephone, a goddess in her own right, so that they both can be married to daughters of Zeus, who could stand in their way? Hercules saves his ass from Hades' punishment, but even despite all of the above bullshit, the Gods still don't abandon him. Not until he murders his own son do they abandon him. He kills his son out of jealousy and out of pride. Now he's old, and his own people despise him, even the gods have abandoned him, so he decides to try and be a hero again. He fails. He's lost everything because of pride. Eventually, he is either pushed or he himself jumps (depends on the retelling) into the sea and drowns. If he was pushed, he was too proud and thought that he couldn't be tricked. If he jumped, maybe he was too proud to believe that his father had abandoned him, or maybe he realised his fatal flaw, realised the devastion that he left in his wake as a result and couldn't live with it.
Theseus represents the worst aspects of Poseidon. He's possessive, temperamental, violent, selfish, impulsive, and all-consuming. He doesn't take no for an answer. He's too proud. If Poseidon is anything, it's a prideful god. See the Odyssey, where he has a long vendetta against Odysseus for killing his son. See the Illiad where he sides the Greeks because Laomedon, the mortal King of Troy generations before Priam, Hector and Paris, promised a temporarily mortal Poseidon great reward for building the walls that would later protect Troy from the Greeks for over a decade, then when the task was done told him that setting him free is the best reward that Poseidon could hope for. Posedion took every opportunity to attack Trojan ships and even sent a blood thirsty sea serpent to terrorise the city. There's countless stories where Posedion's pride is the driving force behind them.
Pride is not all that Posedion is, but it's one of his faults.
Do you understand how significant his surrender is? Because I do and it's driving me insane. He sets it aside and he surrenders to his brother, a brother who once took his powers and made him mortal, the brother who is the very last person in the universe he would ever want to submit to. He does it to save Percy because Percy matters far more to him than his pride.
Posedion is one of the Gods who have grown the most since ancient times. He's no Saint and he will never be one, but he's calmed, he grown, he can now set his pride aside.
It's probably why Kymopoleia was banished (she deserves better from Rick because she is full of untapped potential and ngl hoo did her dirty and I can and probably will rant about it someday just not in this post), because she reminded Posedion too much of his dark side, a side he isn't indulging in as much because he's not conspiring to steal Zeus' throne, because as far as we know he doesn't have any vendetta currently ongoing (other than doing significant damage to Florida when Hera kidnaps Percy in tlh).
We do see Posedion set his pride aside for Percy in the books, in the last olympian when joins Zeus and Co in the battle, letting his own domain face destruction. Kronos doesn't expect Posedion to join Zeus & co because of his pride and he's both shocked and pissed that Posedion has done so. It wasn't part of his plan, he never considered that happening (which makes Hades showing up too so much sweeter). Percy had to ask him to do that. Seeing Posedion set his pride aside without a single word from Percy, who has closed his eyes and accepted his fate? I'M A FUCKING WRECK
