Spirit is a binary Rune Word which works for Shields and Swords at the same time. It is very popular rune word because of its all round good caster properties such as the high faster cast rate, +2 To All Skills, 55% Faster Hit Recovery, Mana and its inexpensive rune requirements. Having it in Shields is a lot more common than having it in a weapon because there are better choices when it comes to weapon. That's not the case with Shields though. Finding a good caster shield is hard and Spirit is considered the Best-in-slot. However, the only base shields that have 4 sockets (Monarch, Aegis and Ward) are Elite and can be found at end game only and have incredibly high Strength requirements for a caster. Particularly beause of the Strength requirement, Spirit Monarch is the most popular option out there as the Monarch has the lowest Strength requirement (156). Apart from running Spirit Shield as your main shield, it is also often used in alternetive Weapon offhand slot, together with Call To Arms, to buff the Battle Order SKill level even further. Another popular choice for Spirit is for use in Paladin Auric Shields with base All Resistance rolls. Using Spirit as a weapon is also beneficial in certain situations because the rune word is cheap to make and it has low level requirement and it can be implemented early into the game when the bonuses are astronomical. Wearing a Spirit Sword at level 25 is a game-changer.