Runestaff
Definition
A runestaff, rune staff, or runestave is a special staff used by magical professions to provide defense. The system treates it as a two-handed weapon, so a shield is not used.
Training Requirements
You must have at least 8 "magical" training ranks per level. Two-handed weapon training is not required for defensive use, but is for offensive.
Advantages
The primary advantage of using a Runestaff is that you don't need to spend any additional training points in its use. Most pure magic users will already have more than eight ranks in magical skills, resulting in essentially free defense. Also, many pure magic users eschew the use of mundane tools such as a sword and shield, and a Runestaff allows them that luxury. Finally, a Runestaff is the only form of defense with which you can parry a ball spell or bolt spell.
Disadvantages
Because a Runestaff is considered a two-handed weapon, it does not provide as much defense as a shield. Specifically, the bonus garnered from a Runestaff in defensive stance is comparable to a shield's, but in offensive stance the Runestaff is significanly less effective. This is because any weapon provides parry defense which decreases in effectiveness as your stance becomes more offensive.
Due to this mechanic, getting a Runestaff significantly enhanced via Enchant Item (925) is often not worth it because the bonus decreases to zero in offensive stance.