Though it is unknown what caused it, the event most commonly referred to as the downfall of the gods ended in an all out war between gods and mortals.
The gods were slain because of the sheer disadvantage in numbers of ability users.
When the war ended the gods had died out and for some reason so had the humans ability to conceive superhumans in families that don't have the blood of one.
Excerpt from A look into the age of the gods.
_____________
"The name's Thea. Last year, I found out that I could generate and control electricity. I was there when an electrokinetic was attacking a mall and had taken a child hostage. I couldn't just sit still so I knocked him unconscious. People filmed it and I became famous and was incarcerated for unlawful use of superhuman abilities. Now that I got out I am getting my licence." she explained.
"Hi, I'm Leo. I started bonding with an artifact 6 months ago, have awoken an ability and am going to kill The Whisperer. Nice to meet you." I of course introduced myself because that's the nice thing to do.
Everyone else introduced themselves too. We all stepped into the teleporter and were blown away by the sight of the main facility.
"This is Olympus." Athena said. We were atop a mountain in front of us was a towering building made from marble. Statues of the gods were in a circle around a Flame.
In front of this flame was a woman, she looked 18 but I had a feeling she was older. Leading up to the door was a row of the greats. They were the strongest and had gotten a place on the highest floors.
The gate itself was a statue of the strongest of them all: Plamya. She was elusive and rarely showed up.
Whilst the others went inside I went to talk to the girl in front of the fire. "What are you doing here?" I asked.
She looked at me like I was crazy. "You talked to me? What do you want"
"I don't know, I just thought you looked like you could use some company." I answered.
Athena appeared next to me. "Hestia, is that you?" The girl looked up at that.
Wait, she looked up at that. SHE COULD HEAR ATHENA!
"You, you have the pen don't you? I never thought it would actually bond seeing as Athena would only bond to someone wise and with a strategic mind." She sounded amused.
"Wait up, you're Hestia. As in the goddess of the hearth. As in an Olympian goddess. How?" Athena seemed to wonder the same thing.
"After Athena died, she was the first casualty of the downfall, the other gods went to war with mortals, I stayed here protecting this flame as is my duty but when they died I couldn't take it anymore. I put out the flame and went on a journey of self-discovery."
"What's so special about this flame?" I asked. "It's the origin of all superpowers. It tended to be found by accident by people until Chaos found it, he passed it on to Gaea, she passed it on to Kronos and he got killed and I was then elected to take care of it. I am the last god so I was the only one who could open this place. I've reignited the flame a few time but until now I've never found a reason to reopen this place." she answered.
That makes sense, it explains the reason it comes in waves is because the flame has to be passed on before it stops burning if it does it disappeared and has to be found again.
"Well, I best enter before Thea and the others wonder where I am." I leave and enter. The first floor is a reception hall that is enormous.
Turns out the others hadn't even finished exploring the first floor. "Leo, did you know we get our own rooms here? And there's an arena to fight, a cafeteria that serves everything from MacDonald's to 3 stars food. We do have to pay for everything so we can't really afford it but still. There's a missions board where we can take on missions and even earn some money."
They keep on talking for some time. Then we decided to test our mettle against each other inside the arena. Thea went first, she was up against someone with superspeed.
The battle started with Thea immediately attacked. The guy with superspeed, his name was Brian, dodged by a hair. He started attacking her by punching her and then running away before she could retaliate. The third time he did this though she surrounded her body in electricity, when he tried to punch her he was knocked unconscious.