Helpless

Helpless, a term often mentioned in game descriptions, is the definition of a creature state that consist of many status effects with certain similar properties.

Helpless "ignores" the calculation from armor class. It allows all incoming attacks against victim to bypass the attack roll (which also bypasses critical hits/misses), hitting automatically, but still subject to magical defenses (stoneskin/mirror image/protection from normal/magical weapons/etc...).

The state is set by opcodes 39(sleep), 45(stun), 109(paralyze), 157(web), 175(hold), and 185(hold). Time Stop causes all attacks against any creature it stops to work this way as well, although it does not actually set the Helpless state on any of them.

A weapon with an internal THAC0 bonus of 32767 works the same way, it has no attack roll, cannot critical, and always hits, barring magical defenses. Arrows of Detonation use it.

Sources & references

 * Most writings are credited to kjeron