Runestaff
A runestaff is a special defensive item designed for use by pure professions. Runestaves are also treated as two-handed weapons for offensive purposes.
Training Requirements
Though any amount of training in magical skills will provide runestaff DS, 8 ranks of magical skills per level will generate DS approximately equal to that provided by training 1x in Two Handed Weapons. Actual training in the Two-Handed Weapons skill is required to increase offensive AS with a runestaff.
Advantages
The primary advantage of using a runestaff for defense versus other training paths is that the skills required are already considered to be a main part of a typical pure spell-caster's training path, and as such come at little to no additional cost to the player in terms of training points. Runestaves are also the only form of defense which provide parry defense versus ball and bolt spells.
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 near-zero in offensive stance.