@Sunbather That's good to hear, always nice to twist old cannon into something new.
Regarding Amon's age I can see two way to resolve the issue. Firstly, we can utilise the one you've already provided. He is new to the lands of Nelhain, or we could use his Lichedom to reduce his memory. Perhaps whilst the process preserves the knowledge the being possessed before death perfectly; it doesn't manage to preserve everything the being learns in undeath. So say every 100 years, Amon and the other Tomb-Liches would begin to forget old memories to make space for new ones, like a computer hard-drive overwriting old cache files but preserving core systems.
That way, he can retain all the basic things he'll need to survive (how to fight, speak, cast spells, etc) but won't have a perfect memory of everything that's happened since he died centuries ago.
As for why he's come to Nelhain, well he could be there searching for an item of magical power, seeking new lands free of the Viyore, etc. Perhaps once there's a few other CS up I could find a more fixed motivation. Right now I feel it best to leave it flexible if that's alright? Just so I can see what the other characters are motivated by, it might allow for a greater interplay between characters.