"additional damage (1 an+mr)" = ?

[Taqwus]

The 12 and 10 (from unarmored illithids and illithid soldiers respectively, if memory serves) dictate how strong the paralysis effect is in terms of how long it Lasts, I believe. Paralysis is a completely different attack form, in that it prevents any unit action (spell, movement, attacking, et al) and reduces defense to 0. It does not inherently cause damage, however paralyzed a unit is, however.

(One might note that fatigue damage, on the other hand, -can- be lethal by itself; additional fatigue damage on somebody who's at 200 points already gets converted to hit point damage at some ratio that I can't recall.)

Thus, when an illithid attempts a mind bLast, the victim has a risk of being paralyzed (the main effect) and also has to make a separate check against a (probably small, although the die roll is likely open-ended) amount of armor-negating damage. This physical damage is small enough that it can take quite a few mind bLasts to kill somebody, and frequently it won't do any hit point damage at all.