[the phantom]

It would be very useful if it was believed the Seer was about to die though.

Perhaps the power of the unknown role is he/she can take the Seer double-dream and have them sent to a random Living innocent on a night of his/her choosing. BUT, in order to make the power work correctly, the unknown has to correctly guess the Seer.

So, perhaps the Unknown believed Legate was the Seer and was about to die, and so chose to use the power. But because the Unknown incorrectly identified the Seer, the power still worked but as a consequence the dreams were sent to a Baddie.
OR as a consequence an innocent dream subject (Mac) was killed and labelled as guilty.
OR if Lottie is lying about the kill, the incorrectly guessed Seer (Legate) is killed and the dream is sent to a Baddie.