I'll go with the assumption that the island/peninsula Daemonheim is presently located on was controlled by the Necrosyrte at least in the Second Age and part of the Third. The second page of Bilrach's Chronicle seems to suggest Necrosyrte involvement in the place and in ending the God Wars themselves. It appears Guthix had help from them, which is surprising after a quick glance at your link. Your historian implies that Guthix and the Necrosyrte should be enemies.

All very interesting. Thanks!