Secret Knowledge is a skill which gives you ability to buy Guidebooks.
After studying on guidebooks you can get certain rewards for killing monsters. Examples:
-You can get
Guraldiy Korr Slime for killing Guraldiy Korr
-You can get
Scorpion Bile for killing Large or King Scorpions
As you can see secret knowledge is necessary if you are trying increase your Juggernaut reputation.
After studying on necessary Handbooks you can use Chaos Elixirs and Amulets also Undead Elixirs
-
Encyclopaedia of Chaos Amulets
-
Encyclopaedia of Chaos Elixirs
-
Guidebook of the Undead
And you need secret knowledge for some profession recipes
-
Treatise on Rare Plants I
-
Treatise on Rare Minerals I
-
Treatise on Rare Fish I
Actually i didn't know what is Sacral Knowledge so i studied a little bit
Sacral Knowledge is a skill for mages. After doing a series of quests for Eldives or Kroffdors you can start increasing this skill. But how ?
When fighting against Eldives or Kroffdors (opposite of which you choose) under the effect Ulleriuni or Irrigudo you can get pages of chronicle tomes I-V. Reading a new page gives you 1 Sacral Knowledge skill.
There are five total chronicle tomes so they can increase your Sacral Knowledge skill to 5. After that you can give 12 pages of the same chronicle to mages of ancient races to get +1.
Sacral Knowledge gives you access for some tasks. And if you are level 14 or higher you can access an instance to. It's other earnings are a mystery to me. So i can't give you any further information about Sacral Knowledge.