Spirit is a cheap Runeword that can be used either in a Shield or a Sword. Requiring four common Runes, Spirit is an affordable Runeword designed with casters in mind. Whether you create it in a Shield or Sword, the mods common to both items are a caster's dream come true. Let's examine how viable the Spirit Runeword is for a caster.
Stats for Shield Only:
Cold Resist + 35%
Lightning Resist + 35%
Poison Resist + 35%
Attacker Takes Damage of 14
A Spirit Runeword in Shield offers very good bonuses to your character's Cold, Lightning and Poison Resistance. (total of + 105%)
However, it should be noted that there are only three Shields in the game that are capable to have four sockets: Monarch, Aegis and Ward. Out of these three, the one with the lowest Strength requirement is Monarch, which requires a Strength of 156 to equip.
That is a rather high strength requirement for casters. If you are planning to reach a Strength score of 156, the Unique Shield Stormshield offers even more Resistance bonuses: + 60% Cold Resistance, + 25% Lightning Resistance, and + 35% Physical Resistance, which is harder to come by then Elemental Resistances . Not only that, but Stormshield also offers 25% Increased Chance of Blocking and the + 35% Block Rate mods, both of which are very important for casters, who usually depend on Blocking to save their lives due to them being more fragile than melee characters .
The good thing about Spirit is that you can equip it at Level 25, while you have to wait until Level 73 to use Stormshield.
Stats for Sword Only:
Adds 1-50 Lightning Damage
Adds 3-14 Cold Damage 3 Second Duration (Normal)
+75 Poison Damage Over 5 Seconds
7% Life Stolen Per Hit
Casters benefit the most from the Common mods on both a Spirit Shield or Sword. A caster wielding a Spirit Sword will enjoy all mods common to both Spirit Shield or Spirit Sword. The mods that are specific to a Spirit Sword will be of minimal use to a caster, since they all rely on actually physically hitting the target, and by their very nature, casters preferring to cast spells rather than wade into melee.
Unlike the Spirit Runeword in a Shield, even a low-level Broadsword requiring a Strength score of 48 can be used to create the Runeword in.
Spirit is a very, very affordable Runeword for low-mid level casters. The mods common to both a Spirit Shield and Spirit Sword really benefit casters a lot. The Spirit Shield lacks a bonus to its blocking chance, something a caster really needs to keep herself alive. The Spirit Weapon is a good caster weapon, even though some of the mods will be less useful, as they require you to actually hit an opponent and thus wading into melee, something most casters avoid.
Still, for all its little detractions, Spirit is a great Runeword for low-mid level casters and can act as placeholders until they find something better.