Before anything, I searched for info about this and coudn't find any threads or forums talking about it. If it was already discussed, I'm sorry for bringing back this again.
So Jari Mäenpää had a band before Ensiferum and Wintersun called Immemorial. This wasn't a side project or even a one man band, since it was his first band and he composed songs with a friend, Oliver Fokin.
Immemorial did a demo in 1997, although I couldn't find info about it being released. Yet, there are a lot of youtube uploads of the songs and the demo, so it would be weird if it wasn't released.
Also it's mentioned that there's a split demo CD with Ensiferum and Wintersun from 1997 with one of the songs from the Immemorial demo, yet it isn't on their discographies.
I think Immemorial should be on the website because it was a band with 2 members and have one demo. Format unknown, but someone must have it somewhere since there are so much youtube uploads. Also in Jari's biography here on the archives, Immemorial is refered as a band, and it's metal enough, so I don't know why isn't it here.
And again, if this band was already discussed I'm sorry, I searched for it and couldn't find anything. :)
Links:
https://www.last.fm/music/Wintersun/Immemorial/+wiki (info)
https://www.youtube.com/watch?v=lvDM-1g03bA (info in comments)
https://www.youtube.com/watch?v=NfHj75v5WnE&t=1s (full demo)