I like that change. It really feels like something that would have been in the game given how fucking dark it is; sounds like you took a page from Xenogears/Saga on that one.
God knows that Thanatos felt flat and undeveloped in the original game and amounted to pretty much the token "I'm bad, watch me do bad things, k bye" antagonist.
Thanks, but the most funny thing is, that I never played Xenogears or Xenosaga.
I only know vaguely a few things from those games though by telling and Tv tropes.
Well, the idea of the Egg could be contributed towards Saga Frontier 2 (partial the design elements from its final boss) and Berserk (man-made Behelit).
However, the idea is not new that villians use a device to become something greater, more powerful or use a cocoon to metamorph into the next stage.
Regarding Thanatos, I decided to make him and Belgar, the masked Magician to one and the same person.
The Dragon Emperor Drakonis/Tiamat was according to lore in fact the true villian of Seiken Densetsu 3, because he is the current ruling Archdemon of Mavolia and chosen one of the Thanatos, spirits resembling masks which serve the dark echoes. . Anyone touched by them becomes a Grimsly/Mavole, demons and other kind of monsters so to speak.
Depending on which main character you chose, the other two fractions are killed by the respective main antagonist for the character.
Dark Prince Strout from Lorimer, who ursurped Anise the former Archdemon (which is responsible for Mavolia and the dark echoes), was left in a severely weakened state, despite succeeding and was sealed by the Goddess of Mana.
Drakonis killed Stroud and Belgar later Anise finally to become the reigning Archdemon.
I assume, that Belgar who became corrupted by the dark arts and died by Drakonis' hand, came back as the Mavole named Thanatos.
The reason why he goes by the name has certainly to do with the fact, that the Thanatos gave Belgar the means to return to the world of the living, thus respecting them greatly as he himself still wears a mask. He however needs a body as told in the original, because his dark Lifeforce is very potent and easly destroys uncompatible hosts.
Interresting to note: Mavolians who fall in love with a human, are punished upon return to Mavolia with amnesia by the Archdemon.
It explains quite nicely why Bigieu goes by her alias Isabella the entire time in Sword of Mana.
I will join forces eventually with The Seikens, so I can create with their help a lore friendly expansion, creating stronger ties to the prequels and sequels as a result, without harming the canon.
The lore will be implemented in various ways, by books the player can read and by NPCs, for example a Ministral/Poet/Bard ala Romancing Saga.
If anything goes according my plans, the expansion will rival the extensive romhacking scene resolving about the Romancing Saga 3 hacks. And if we are daring enough, we might even include blood graphics like those from RS3.
Remember though, I cannot program or use ASM to do all of these by myself.
Only RedScorpion is capable of changing the ROM itself of our current team.