I was going to post something else but can't remember it.

Anyway, Mar keeps running away every time I talk to him. How do I stop that so that I can finish that quest?

Ah! And why does my armor stat go DOWN when I wear armor class boosting items?

In D&D, a lower character AC actually represents better dodging/defense. I believe -10 is the limit in PS:T.

You need to speak to various other NPCs to finish the Mar quest. It will tell you in your journal who to go to next. I'm pretty sure you can check off the quest (eventually) without ever speaking to Mar about it again.

The armor in this game works differently, lower = better. Not sure why.

THAC0 stands for To Hit Armor Class 0. If (as is the case with all the base classes) your THAC0 is 20 at first level, this means you must roll a 20 to his an Armor Class of 0. To hit an armor class of 1, you must roll a 19. I seem to recall striking an enemy wearing full plate armor and from a direction protected by their shield (if they have one) comes out to an AC of 0.
The base AC of everything you meet, in general, is 10. Therefore, at a THAC0 of 20, you hit unarmored opponents with a dexterity of 10 (the average of 3d6, the roll for 2nd edition stats, is 10.5, so 10 is average) about half the time.
Armor class is modified by dexterity (high or low), armor worn and shields carried, natural armor (particularly tough skin, such as that of a dragon), magical effects (armor/shield spells, magic items which improve AC), circumstance (cover, for example), and other odd things.

Yes, this system is bass ackwards. As a system for arbitrating hits and misses, it predates video games by a bit and is a step away from D&D's original inspiration, war game simulations involving tactical deployments of troops and vessels. They got rid of this system in 3rd Edition, but it would have been taking significant liberties with TSR's system if they tried to get rid of it entirely here.

it says to go talk to mar.

I like this THACO system a lot. Would probably be a real drag to figure out with pen and paper (have never played it), but in a computer game it works well. Main thing is that the lower AC and THACO is, the better. You'll be harder to hit and it's easier to hit others.

THAC0 vs. Armor Class actually works almost exactly like the Base Attack Bonus vs. Armor Class in 3rd edition. You take your THAC0, you add the AC of the target (or subtract if it's negative) and any modifiers, and that's your target number. Roll a d20, if your roll is your target number or higher, you hit! It's actually pretty straightforward, but it caused a lot of confusion for players, especially ones that weren't good with math, because sometimes you were adding modifiers for the target number, and other times you were subtracting. People also tend to have a harder time subtracting than adding, unless they're math people. (One of the suggestions for cashiers calculating change is to "count up" to the change, rather than subtracting the change from the money tendered... because adding is easier, even if it's the same.)