If the assassination is taking place outside the Palace, the threat model, then, is an attacker who has a plan for making off with a gem containing Abrogail Thrune's soul, or with the Crown of Infernal Majesty, or less likely kidnapping her living body.
The full details of Abrogail Thrune's security entourages outside the Palace would be lengthy; and, of course, heavily classified.
But a primary element of her personal Security is that Abrogail Thrune will have a Status spell cast upon herself; and, if she's injured or dead, the person who cast that spell will be in telepathic range of Gorthoklek, who will promptly Greater Teleport in - not to that exact location, as will obviously be Teleport Trapped by at least one side of the security game; Gorthoklek will Teleport in above and dive quickly.
(Abrogail Thrune does sometimes appear in a place without heavy Security - life would be less fun for her, otherwise - but when she does, she goes Mind Blanked and never ever lets it form anything resembling a predictable pattern. The one Competent Security on her personal guard invariably tears her hair out anyways.)