First Era- The First Era was the one of Mithra, the Goddess of Absolute Good and Evil (Zenaku-Nigen).  In her world, all sentient beings were split into two types of beings, one labeled as purely good, the other as pure evil.  This is perhaps the most expansive - in terms of scale - of the worlds and is the base for the web novel Avesta.  I'm not going to spoil Avesta's particulars, but I am going to make note of a few key points that are made in the Pantheon retcon.  As the narrator, Kouha, says, this was a world were more people died constantly on a daily basis than in any of the other eras.  Evil's people were shameless and utterly merciless but lacked any malice, whereas the side of Good were full of hate, rage, and bloodlust.  This created a situation where peace was literally impossible, since neither side was capable of understanding or even wanted to understand the other.  It was ended by Muzan, the second era's god, who was a member of those who referred to themselves as Good.  

Second Era- Muzan, being a product of Mithra's world, was quite naturally also a product of the endless wars of that era.  He was a man devoid of shame and regret, his only desire the annihilation of evil, and as a result, his law became one of the Shameless and Soulless (Muzanmuki), a world of evil beings who devoured evil.  Every being in his world was born with Original Sin and had to live in a world of suffering.  All beings in his world were utterly shameless, doing what they wanted like animals, only to be slain and devoured by those stronger than themselves.  Despite this - or perhaps because of it - it was a world that repeated cycles of progress and glory followed by rot and collapse.  It was ended by Neros Satanael, also known as Myoujou (The Morning Star), who utilized artificial angels and devils to 'purify' the world.  This was the world of Paradise Lost.

Third Era- Myoujou, driven by rage against the evils of desire in Muzan's world and an absolute Pride and belief in his own righteousness, decided that the reason why conflict occurs is because there were differences between others.  His world was one of gradually removing those differences, gradually heading toward the creation of a gestalt that would be devoid of individuality in all its forms.  Unfortunately for him, the very nature of his world and the methods he used to overcome Muzan (turning back time at one point), made his own defeat inevitable.  In a world where everything was known to everyone, so was born a god who desired nothing more than to find the unknown.  Myoujou, whose personality was very close to that of a machine and incapable of selfish desires, upon realizing that his world was flawed, simply stood down without a fight, resulting in the beginning of the Fourth Era.

Fourth Era- The world of Suigin, Mercurius.  It is also the world of the Dies Irae visual novel.  Mercurius, being the product of Neros Satanael's time-warping as well as his world, was a man full of contradictions.  His world was an eternal loop, Eigou Kaiki.  From the very beginning, Mercurius was like a man born old, and it didn't take long for the contradictions inherent to his nature to drive him mad.  Obsessed with the unknown, but also split between the desire to be destroyed by his best friend and the desire to see the woman he loved replace him, Mercurius repeated the same loop again and again, acting out his comedic opera using the characters of Dies Irae as both audience and actors until he got the result he desired, one where  his beloved goddess, Marie, reached the Throne and he was cast down.  As is noted both in Pantheon and in Kajiri Kamui Kagura, this was by far the longest era, stretching across hundreds of thousands of years.

Fifth Era- This is the era that split the canon, the era of Rinne Tensei (The Wheel of Reincarnation) where all beings were watched over by a loving goddess as they were born, lived, and died, experiencing innumerable lifetimes.  The goddess of this era is the sole deity who in no way interfered with the mentality or fates of those who lived under her law.  With her gentle hands, the goddess even salvaged her enemies and allies from the Fourth Era - including Mercurius, Reinhardt, and Ren - and had them exist in the world of her law.  This is the sole era where the deity was capable of allowing coexistence with other deities.  However, even this law had a fatal flaw.  Due to not having had the experiences of Rea's path and being too close to Ren as her lover, Marie never embraced the ability to eliminate others from her world, so when a cult of nihilists decided to rebel against her law, she did nothing.  This cult was driven by the ideal that 'This is my only life, I care not for others, for that is what it is to live.'  Unfortunately for the cultists and their leader, they didn't have the personal power to challenge the Throne, but they instead decided to create a toxic bomb, in the form of someone who could... a malformed child driven by the most extreme form of narcissism but with the potential to reach godhood... Hajun.

Sixth Era- This era was easily the shortest of the six eras.  At the moment Hajun (Tengudou, Metsujimessou) struck down the goddess of the Fifth Era, her lover, Setsuna (Ren) gathered his few surviving comrades and unleashed the Law he had previously rejected, holding back Hajun's law in hope of finding a way to crack through Hajun's world.  Hajun's law was quite simple... there need be no one other than himself.  If his Law were allowed to take full control, all life would perish within days, madly slaughtering one another until only Hajun remained.  It was only through a miracle of fate and the delaying actions of Setsuna that this world never saw true completion.

Seventh Era- This era... doesn't really have an explanation, at least so far.  The ending of Kajiri Kamui Kagura indicates that this was a world that contained aspects of all the previous deities, but Masada was pretty vague in his descriptions.  Technically, Pantheon begins in this era, but the failure to complete Pantheon led to this part of canon never being fleshed out.  Originally, this was intended to be the final canon world of this canon, but Pantheon's retcon twisted that around a lot.