As for me a multiclassed cleric-thief is the most powerful mellee-oriented thief combo after fighter-thief. Especially when it comes to backstabbing. Yes, backstabbing. Who told you he can't? Well, alright, clubs and quarterstaffs don't suit a "stab" conception actually, but game rules allow this. So why shouldn't we? This is the first thing to mention.
So far as it concerns backstabbing, strength is our best friend. Not only for thac0 bonus, but for additional damage as well. Say half-orcish cleric-thief with 19 str recieves +7 to damage, and what is most important, this bonus is applied before backstab multiplier. Thus in the very beginning of SoA such character with a simple dagger will hit for (1d4+7)*3=24-33 damage! This is the second.
Cleric-thief's modest offensive powers are very well compensated with divine magic.
Draw upon Holy Might, Holy Power, Champion's Strength will always come in handy.
But what grants a cleric-thief true power is the Righteous Magic spell. Along with a significant str bonus it makes all your hits deal maximum damage. That is really astounding. I'd even call it an early Assasination ability (5th lvl spell - 9th lvl cleric - only 450k (!) exp. needed). This is the third.
And to end up this munchkeen image we'll buy some underestimated weapon: Staff of Striking, which is noticable for +9 damage bonus for each hit.
Now let's count this up:
Say we have half-orcish cleric 11 - thief 13 (1,4kk exp) with 19 str and the Staff.
Righteous magic will give him additional 3 str - 22 total. That is +10 to damage.
(6+9+10)*5=75 with a single hit. Be sure to go invisible to repeat the trick if needed.
Of course this is not an ultimate solution and has its drawbacks:
Even with a thac0 bonus for attacking from invisible state one can miss. Fotrunately this doesn't happen often. On the other hand, where there're critical misses, there are also critical hits
Staff of Striking is a non-enchanted weapon which limits its application. Though it still will work for many enemies in SoA. Another thing about this weapon is that each strike consumes charges, which are limited. This is not a problem though if you use it only for backstabbing and you can always sell and re-buy it to prolong its usage.
As for ToB, there you'll continue your righteous
with a glorious Staff of the Ram (as depicted).
Why am I writing all of this?
- Just to show that this combo is already incredibly powerful and needs no futher conversion IMO.
P.S. Writing this I imagined a futher cheesing such character with a recently released Multiclassed Multikit Mod. How do you feel like Priest of Lathander - Assassin multiclass with a x7 backstab multiplier and an additional attack from Boon of Lathander?