i played a assassin/mage (the ultimate shadow mage
) but i never got to finish it because of a pc-crash.
but i dualed as a lvl 10 assassin just because ad lvl 9 i didn't have enough points in Hide in Shadows.
i don't have Tob so i don't know much about the high lvl abilities.
so dualing around 9/10 should be best because then u have enough points to put in HiS or the other thief abilities (but as a Shadow Thief u should only be backstabbing and poisning the hell out of everyone so u don't need al the rest