So, after reading the prologue, I decided to make a list of all the facts I could squeeze out of it.
1. Ruin has control over an Inquisitor's body and almost-total control over their mind, when it is possessing them.
2. It is possible for a Steel Inquisitor to resist Ruin's control over their mind for brief periods of time.
3. There is probably no way to stop Ruin's ability to control an Inquisitor other than death.
4. The wounds caused by an Inquisitor's spikes do not bleed.
5. The spikes give an Inquisitor power.
6. Inquisitors have access to Allomantic abilities. ("Everything was outlined in fine blue Allomantic lines . . . ")
7. Ruin likes to hear his prisoner's scream and enjoys causing pain.
8. Inquisitors have steel spikes through their eyes, and at least one brass spike in their chest.
9. Making (Or possibly just enhancing) an Inquisitor requires killing a sacrifice.
10. Ruin is free, but cannot directly affect the world too much while the other entity (Preservation) is free.
11. Vin freed Ruin, who is now looking for something to increase his power. (Something to seal away Preservation?)
12. The spike used in this ceremony must be driven point-first through the Keeper's heart into the Inquisitor.
13. The Keepers are being interrogated about the Synod, then used as sacrifices.