spirits

Spirits are minions of the Nine created from their own essence to be eyes, mouths, hands and sometimes fists in the mortal plane.
All the Nine use spirits but they don't all use spirits the same way. It is widely believed by theologians that while the Nineare truly mighty, their power is not infinite. It presumably takes a tiny portion of a deity's power to empower mortal spell casters or create spirit minions. It is noted that deities that have relatively large number of mortal spell casters usually have relatively few active spirits and deities that have relatively few mortal spell casters tend to have many of active spirits.
Hallisan, Khemra, Phidas, and Mera all have comparatively large priesthoods and comparatively few active spirit minions. Of these four, Mera has the most but she seems to have sacrificed quality to gain quantity commanding very few spirits of great power.
Greymoria, Maylar, Nami and Zarthus have relatively small priesthoods and relatively numerous active spirit minions.  
Like with many things, Korus lies between the extremes of his siblings with a modest number of spellcasters and a modest number of spirits.

Spirit Weaknesses

Nearly all spirits are repelled by a line or a circle of salt . Weaker spirits cannot cross a line of salt at all. Stronger spirits can do so but it causes great pain and weakens them. This is because salt is the essence of Turoch's  blood made and Turoch hates the Nine, so it makes sense that his blood would harm the Nine's servants.
Nearly all spirits are vulnerable to cold iron, otherwise known as wrought iron. Now technically the vast majority of the iron in the medieval world is/was wrought iron. For the purpose of Scarterran metaphysics, cast iron and hardened iron (proto-steel) is not considered cold iron. A spirit’s weakness to cold iron is not extreme. Cold iron weapons do bonus damage but it is far from a guarantee of instant victory.
Spirits are not permanently harmed by simply touching cold iron, but they find the process painful.
It is unknown why spirits are vulnerable to cold iron exactly. The most popular story is that the iron ore found in the ground throughout Scarterra is actually Turoch's fossilized bones.

Some spirits have unique weaknesses not common to spirits in general. Generally speaking more powerful spirits are more likely to have such weaknesses than weaker spirits. These weaknesses tend to be highly unusual.
A spirit might be unable to harm a mortal wearing his clothes inside out, a spirit may be harmed by the crowing of a rooster, a spirit might be harmed by hearing its name backwards. The sky is the limit.

How are spirits created?

Most commonly the Nine take a very tiny portion of their selves and create an independent being out of their essence. These are called exogenic spirits. Less commonly the Nine take the soul of a deceased mortal whose actions and piety pleased them and infuse them with a tiny portion of their power. These are called exemplar spirits.
Exogenic and exemplar spirits are very hard to tell apart. Most Scarterrans are not even aware of the distinction between the two.
Either type can be very powerful or very weak or anything in between. The main difference is that exemplars have free will and the full creativity of a mortal while exogenic spirits are usually very single minded and oriented toward a single task or goal.

Spirits connections to others

Spirits are very similar to the Fair Folk in many ways.  They are both repulsed by salt, harmed by cold iron, and subject to odd superstition based personal weaknesses.  It is commonly believed that the Fair Folk were the original spirit servants of Turoch and the Nine copied Turoch's spirit creation methods when they inherited his creation. 
Spirits are vaguely similar to elementals in that they both are native to another plane and frequently visit the material plane.  They both can be summoned and banished by mortal conjurers.  Their similarities mostly end there.
Void demons are sometimes referred to as "the spirits of Turoch" even though this is not very accurate.  Void demons are more accurately viewed as an antithesis to spirits.  They will often target a spirit before targeting a mortal and it is rumored that a spirit killed by a Void demon is permanently destroyed and cannot be resurrected by the Nine  unlike spirits with most deaths.  Spirits are commonly observed to be far more cautious when fighting Void demons than they are fighting almost any other foe.