Well I can't wait to learn what the hell is going on.

Possible spoilers (nothing that isn't in the demo obviously)

You're locked in a cage along with many other prisoners in cages. You are being sacrificed one by one to some monstrous magician known as Merlin (maarin), when you see the guy next to you start freaking out because he's next. When the skeleton servants come to take him away, he uses magic to burn them and burst open his cage, but he is caught by Merlin and sacrificed.

From his burned out cage you find a talking book which takes you as its new owner, and he tells you that he is a diary of a magician. By reading the diary, you can enter into the memories of this magician and do what he did, starting with his trials as a magician. This is supposed to help you become a magician and break out yourself, as the last guy (almost) did.

The very first entry has you fighting with Merlin against a massive snake creature, and the only thing you know about your own character is that Merlin calls you his "partner". This ends after Merlin kills the boss using sacrifice (a bit misleading, since it doesn't actually kill you, just mangles your body)

After that, you start from the beginning of the writer's magician days, working through each entry, learning more about his partner at the time Nimue (also known as Vivian in Arthurian legend, apparently, just googled this) who is a heartless magician who sacrifices everything she kills. Monsters all come from ordinary creatures who have been corrupted by magic, such as mice, cats, or humans in the case of larger monsters. When the monsters are defeated, they show their original form, which gives you the chance to let them live (this heals you, gives you their life energy, and is the "good" choice) or sacrifice them for your own gain (this regenerates your abilities, gives you their soul, and is the "power" choice). The official stance of Avalon, the primary organization for magic users, is that all monsters must be killed (sacrificed) for their sins. Nimue follows this rule, but her arm is evil and corrupted, and pains her from time to time. You (i.e. the diary writer) comfort her, which leads her to open up to you slowly.

However, this only causes pain for both of you, as the truth of the magician's test is that all who attempt the test must sacrifice their partner to prove their magical and mental strength. The last mission of the prologue is you killing your partner Nimue. She forgives you, but according to the diary itself (Librom) this greatly affected the writer of the diary (If you go too far in the pages of the diary, it says "my memory is missing in places, It seems the girl in my right arm intends to take over my body" among other ominous phrases)

That's about it for the story. From there it's just the 4 missions as a member of Avalon, which don't really have any story to them. It seems that it connects to Merlin somehow after that (the three chapters are Nimue, Avalon, Merlin), but that's not part of the demo.

If I messed anything up or this is hard to understand, let me know and I'll double check/correct.