Pyros is a fully functional star, albeit with a surface of harsh volcanic rock as opposed to burning gas and plasma. It is totally inhospitable to humans, as its intense heat is completely forbidding to most other sapient species in the universe.
It is covered in sharp volcanic structures, volcanoes and mountains covering almost every horizon with rivers of lava carving infernal rivers through the environment.
Pyros was once ruled by a fair king of the Solar Dynasty, however he was overthrown by Apollo and his forces after Apollo came to possess the weapon Ascalon. According to Sella, he came to power overnight and no rebellions would have a chance to rise against him.
As it was later revealed, Ascalon's arrival on Pyros was the fault of Eon and Professor Paradox. Eon stole Ascalon from Azmuth and brought it 30 years into the past as he fought Paradox for it. During the fight, Eon dropped the weapon but Paradox was unable to retrieve it as their fight took them elsewhere. He could not return due to the Time War.
During the early years of his rule, Apollo declared war against the planet Piscciss. After years of futile warfare the rulers of Piscciss were able to reach a peaceful agreement with Apollo's advisers, where they would attempt to convince Apollo to call a cease fire and let this be a 'warning' to the people of Piscciss. Luckily for both sides, he agreed and took pride in what little his war had accomplished.
Recently, Sella was able to escape from Pyros (a feat she claims to be the only one to achieve) and fled to Earth to recruit the help of Ben Tennyson, who she had heard of through his extranet show. Pyros is currently at war with the Earth. While the Plumbers assumed this was to retrieve Sella, Apollo has since disproven this.
The war with the Earth ended with the death of Apollo. When the Covenant of Fire heard of this, they appointed Ignis as the new ruler of the planet as she had previously been promoted to Apollo's second-in-command.
Pyros was the first stop in Ben's galactic road trip in the episode From the Ashes. In the absence of Apollo and his army, an uprising of a rebel faction known as the Starlit Sky has uprooted those that remained to keep order on the planet. Ben arrived on the planet only a few weeks after Ignis to find the planet in turmoil. Seeing Ignis' rule as a continuation of Apollo's tyranny he fought alongside Chief Embel to liberate the planet.