Why did Daniel drink the Amnesia potion?

[Jestr]

Very strange indeed. Why steal his journal, rip pages from it, and then put them at different locations? Was the responsible person (or monster) trying to give Daniel his guilt back so that he will lose the will to continue on his little quest?

This is only a guess.. but maybe it was Johann (Weyer)? Alexander seems to be (or was) in contact with him, even though Johann is in "the other world". The letter about the ingredients reveals that Alexander and Johann had a conversation about releasing Agrippa. Maybe Johann saw another option in Daniels quest to kill Alexander. Among all those unearthly events, it wouldn't be that unplausible.

Spoiler below!

Weyer taunts me from the other Side. I trusted him with my true reason for my efforts but still he insists on me releasing Agrippa. How an enlightened man can show such lack of compassion in a seat of power dusgusts me. I can't bring myself to part with Agrippa as he is and has been for centuries my only link to the worlds beyond.

Weyer claims he tries his best to release me from my banishment, but he needs me to give him Agrippa frist so he too can help. If he could gurantee success, i would happily oblige, but how am i to part from him if my return would be denied. I know what they are capable of, i have seen their deception.
Optimism is a most hopeless feeling, but i must retain it. I must prepare for his release.

To release Agrippa without killing him, Weyer told me to feed him a tonic made from paralyzer, vitae and Tampter.

[...]

I will have to adress Weyer with this as he will have to supply me with a host. Also a properly prepared well should suffice to contain it.