Oh really? And what about when one of the priests uses a spell of true seeing or a magic detection? Eos is an important man, he would likely be subject to many security protocols. It's very risky to do this without proper planning. Besides, I have a better idea.
Eos will likely be involved on any attack on the Nightsong Guild, as would the riled-up citizens. Pretend to help the forces of Sirramar attack the Nightsong Guild, than kill Eos in the confusion of battle. We might even be able to take some of the Nightsong Guild's treasure, especially if we had some way of smuggling it.