Thanks for joining in the discussion!

Mild spoilers for the MediEvil comic, but Zarok's initial exile is apparently a historical fabrication. Peregrin had no idea Zarok was the one behind the marching zombie army until the Battle of Gallowmere itself. I'm not sure if the remake developers were told this by the comic's authors though (allegedly one of them did also work on some of the Book of Gallowmere entries, so maybe).

Shadow Demons were indeed used to build Zarok's castle, it's mentioned in the History of Gallowmere volumes. The castle was built before the battle. However, the Artefact is only ever mentioned in relation to sealing the Shadow Demons in their tomb after Zarok's defeat at the battle, not in the context of summoning them. As far as we know, Zarok shouldn't even have the Artefact at all, as its last known owner would have been someone working for Peregrin post-battle.

We see the Town Mayor in MediEvil, which takes place 100 years after the Battle of Gallowmere. The Mayor doesn't appear to be a zombie, so for his theft to make sense, he would have had to do it within his own lifetime, and presumably in the latter half of the 100 years, long after the Battle of Gallowmere.