Day 5: Spent a lot of time implementing a system for monsters to perform actions (incl. casting spells), with associated AI. Added an ending. I ended up dropping item/inventory/levelling support as it unnecessarily complicates the game.
One unintended consequence of the possession mechanism in the absence of levelling is that all critters should have about the same stats, otherwise the player gets killed very quickly. If a monster is overpowered, you have to make sure a counter-monster is present in the level to bring back balance. This makes it very easy to create frustrating experiences. So it may be a mechanism more suitable for short games.
What do you guys think about it?