I don't know how much you're done with the story, so I'll spoiler the thing.
First you must understand that the Royal Army and the Pontiff's Court are different entities with different agendas. The Royal Army answers to both the Royals (Lords and King [dead] ), and they also answer to the Pontiff's Court, whereas the Pontiff's Court has it's own army, magicians, and inquisitors.
Even magicians like Jaralth of the Magician's Guild who is not part of the Pontiff's Court is in league with them, or at least has knowledge of some of their secrets and keeps them ('cause he'd be chopped to pieces if he talked). To my knowledge the two magicians with no relations to the Pontiff's Court are Brynn and Riley (Though Riley might have his run ins with the Court in the past).
Brynn, in his youth found out that Tieve was the oracle of the prophecy, and since he fancied Tieve he tried to summon Morrighan so she wouldn't be sent body and soul to Erinn. At that he ended up summoning Glas Ghaibhleann and the beast nearly killed Brynn if not for the intervention of Riley.
After Riley sealed the beast, the Pontiff's Court attempted their own version of the summoning ritual which ended just as badly tearing dimensions apart and summoning Vampires into the world. So the Royal Army was not involved, and have no knowledge of the corruption of the Pontiff's Court.
Albey and Heide are said to be the same in the lore. Same with Ben Chenner being a volcano (but only while Elchulus is asleep), when you face Enzo it's in the same place you face Lugh, the difference is that Elchulus was still sealed during the Enzo fight, but was awakened during the fight with Lugh. After Elchulus was released the vocalno turned back into being a normal mountain. This is also part of the lore.
Alright, here is what really happened, S1 happened, then a mysterious voice tell the player that there is still a side of the story they don't know, and that they should go through the same events but this time without interfering with Keaghan and Tieve, which is why they are back in Season 2. Regardless, fate couldn't be changed and Tieve and Keaghan still became Cichol and Morrighan. Of course, Elchulus was still released, with his release Erinn descended, time stopped due to his influence.
However, most likely due to Cichol, Erinn is incomplete, which is why there are 2 Brahas, and the same applies to Ancient Glas. When Brynn is taken to check the summoning remnants in Ben Chenner, he identifies the magic as his own, or rather, magic he had been studying to summon the goddess. Which according to Season 1 he did, he even states in S3: "With Erinn this close, it might work this time", and Nyle even asks "This time?".
Basically time condensed, which shouldn't have happened according to Muir, in contrast to Braha however, only 1 Glas exists in Season 3, the summoning still occurred, but this time it was on the Ben Chenner ruins, the summoner was still Brynn in Ainle, but because of condensed time, Glas was on the Ruins, Riley never tackled the beast, and Brynn never became bound to the lab.
Now, the reason no one remembers Tieve or Keaghan is an effect of Elchulus' time fuckery too, whenever someone dies, they become forgotten, like when Marrec, Aodhan and Riordan died, no one remembered them, the same happened to Ceara when she stayed behind to draw some maps for the royal army at the Ben Chenner summit (they were all slain by Lugh). Tieve and Keaghan are not dead per say, rather they ceased existing when they became their respective gods, but technically, they died, so no one remembers them too.
As it was explained by Macha and Riley, memories have a bit of a fail safe to them, once something is forgotten due to Erinn, the memories fill themselves in order to fix the void, this gets harder when more inaccuracies occur, and then the person either remembers or they go mad. Brynn almost fell to madness after the player defeated Glas, but he managed to remember Tieve and the later events of S1, how he guided the player and Keaghan to Heide, how he wanted us to save Tieve, though it came back to him slowly and painfully. This is the reason Elchulus needs to be sealed again, Erinn is collapsing, which is why Heroes are needed, Lugh was the last Hero, and now the player and Milo are considered the next Heroes, they are the ones supposed to remember and help the rest remember, and the ones to seal Elchulus (though, their fate according to Muir is to be forgotten, as only the name of the dragon is remembered).
Unrelated but still somewhat relevant, Macha said she sped up Erinn's collapse somehow, and Muir noted as well that Erinn was collapsing faster than it should.
My scources are "writting down every last detail that is spoken, or seen. Every last dialogue line from season 3".