I wonder, since DY wasn't always known by that name, Toland has extensive disturbing knowledge of the darkness and hive magic (like was used in thorn) And nobody knows what happened to him.
It's a Hail Mary, but I'm not sure there is evidence to the contrary.
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No. Aside from them being different classes. The last known location of Toland was near the Deathsinger. Dregen Yor is confirmed dead. There's no guarantee that they didn't exist in the same time frame.