Kaiser's Last Stand...

[Endgame]

More like Demon Slayer broke his shield, which allowed for Mercedes to damage the Black Mage. Then Luminous shows up to complete the seal that Freud was working on while Mercedes was distracting him.

That MercedesxDemon Slayer wallpaper makes sense now.

[Spadow]

Where does it say when this take place?

If you watch the tutorial of Luminous which is in the past, you will see Magnus with a broken horn.

[Moonlapse]

I'm going to name Magnus "El Suavo".

[Endgame]

Yep, and he has different sprites for before and after losing his horn.

So, he did whatever he was trying to do on Nova, got stopped by old Kaiser, fled to the Maple World, joined the Black Mage's army, backstabbed and stole Guwara's powers, then..... disappeared for hundreds of years?

[Baklava]

Yep, and he has different sprites for before and after losing his horn.

So, he did whatever he was trying to do on Nova, got stopped by old Kaiser, fled to the Maple World, joined the Black Mage's army, backstabbed and stole Guwara's powers, then..... disappeared for hundreds of years?

Maybe Kaiser's attack was so powerful he was thrown into the Maple dimension, then the Black Mage found him near-death and offered to grant him power in exchange for eternal servitude.
Magnus is mentioned in Phantom's quests, but I don't remember what it detailed.

The next update after Angelic Buster is titled "Magnus' Rebellion" and since we already know he rebelled against Pantheon, perhaps his next move is against the Black Mage himself.
They did report that there will be an enemy more powerful than the Black Mage.