Via the new building to the right of your Castle you can acquire extra combat boosts. To get these boosts there is a two step process, first you need to Activate a Tome (with Curiosities Guidebook) and with an activated Tome you can Craft a Curiosity (requiring Copper Essence and for higher level also Ebony Essence).
The setup of the Curiosities are very similar to the Coat of Arms. There are 5 grades which can be refined to a higher level the higher the grade is. Each grade consists of 4 types. There is one that has Troop Attack, Health, Defense benefits, one that gives infantry attack/defense/health, one that gives cavalry attack/defense/health and one that gives distance attack/defense/health. The total boosts provided at the highest level are 80% for the troop specific boosts and 22.86% for the Troop Attack/Defense/Health boosts.
The Curiosities are placed in 8 slots, two of each type. Note that you only need to Activate one Tome of each type with which you can Craft Curiosities for both corresponding slots.
The Curiosities cannot be enhanced, but from Purple onwards they can be upgraded to the next grade (color).
There are three items, Curiosity Guidebook, Copper Essence, and Ebony Essence. Curiosity Guidebook is required to Activate a Tome, Copper and Ebony Essence to Craft a Curiosity. Ebony is not required for Green and Blue, it is for Purple, and I assume also higher and for upgrading Purple to higher level.
The new Night Siege event gives as rewards for points Curiosity Guidebooks and Chests and as rewards for killing Bonus Creatures you get Chests as well. These Chests can contain Copper or Ebony Essence.
There is a new Shooting Gallery field with mostly Copper and Ebony Essence as rewards (some contain Curiosity Guidebook, but these are in my experience relatively easy to get plenty of via the Night Siege.
When you reach top 100 on the ranking for the Creature Arena event you can select 1000 Copper Essence or 200 Ebony Essence as possible rewards. Actually this is for rank 51-100, higher allows you to select even more.
In the Goods Exchange there is a new section to request Coppper and Ebony Essence.
For activating a green, blue or purple Tome you need 5, 25, or 100 Curiosity Guidebook respectively.
To craft a green Curiosity you need 100 Copper Essence, to craft a blue Curiosity you need 600 Copper Essence, to craft a purple Curiosity you need 1500 Copper Essence and 200 Ebony Essence.
Curiosities are already at their maxed level when crafted, they cannot be further enhanced, but from purple onwards they can be upgraded to the next grade.
As above analysis points out, the difference between the most basic Curiosities and the best Curiosities is quite small, this makes it very attractive to first work on a full set of green Curiosities, and only afterwards start focusing on higher grade Curiosities. Note that you can dismantle Curiosities for Materials, which means you don't lose much by first going for lower level ones (although I don't think there is a way to get the items used to activate a tome back).
This feature is unlocked at Castle 17 and the slots to attach the Curiosities are unlocked on Stronghold levels 17 to 23.