Found Sigil at 28.58 in Stromgarde.

Once you enter stromgarde, take the first road to the  right, passt a few tombs and guards, continue right, cross the bridge, and he is right in front of you in the house.

Beware of mobs hiding on both sides of the path up to the house.

when you attack the prince, you will aggro 2 of his guards aswell, so be prepared.