Special Abilities: 7x backstab, Poison (apply on any weapon), all cleric abilities
The assassin's poison will kill anything that you can successfully hit, whether his weapon can hurt the enemy or not. This can work wonderfully when your weapon normally would not do you any good (say you were fighting Kangaxx), and your poison would then do all your work for you.
As a cleric you cannot access thief’s weapon selection, however you CAN backstab using staffs (I agree, it is rather odd). The Staff of Rynn (+4 bonus) is a good weapon, and it is in your bag of holding, if you start a new game in TOB. Staff of Striking, or Staff of Thunder and Lighting is also good. Personally, I also prefer to use summoning staves and let the summoned creatures fight for me
Sanctuary works wonders, when combined with other spells (like Blade barrier). You can continue to summon new monsters until the enemy is completely exhausted. Plus, as an EVIL cleric, you can convert undead, which is usually much more useful then destroying them. Sometimes my own Skeleton Warriors turn hostile on me, but I can immediately convert them back.
Yes, you are rather weak fighter, but with the right equipment and weapon selection you can became a juggernaut of destruction. And I will argue that the Thief and Cleric skills are most useful in RPG, unless your playing style is ‘Hack, slash and destroy’.