First, you'll need to have already completed the main storyline of Hades 2, by defeating 
Chronos multiple times, and enacting the plan to vanquish him with the help of Incantations and your secret ally.
How Strong are the Hidden Keepsakes
These three Keepsakes have immense power and incredible potential to make you runs far easier. You have to give up control to the Fates, but in exchange you receive huge buffs to the Boons you are offered.
The go to strategy is using 
Jeweled Pom in the first region for Boon Pom +levels, and to start with a 
Hades Boon. Then, swap to 
Calling Card in second region to start rarifying, 
Time Piece in the third to rack up money for final shops, and it's players choice for the final region Keepsake (aside from Olympian Keepsakes).
Beyond their strength, these Keepsakes also offer unique builds that are virtually impossible without them! Have fun trying out new combos, and having the power to make any Boon you want Heroic.
The Fates Whim
Each of these hidden Keepsakes require you to be at the "Fates' Whim." This means you can't use any Arcana Cards or Keepsakes that control your build. All God Keepsakes are out for the run, and you can't have 
The Fates (XXI), 
The Champions (XXII), or 
The Enchantress (XVI) equipped.
Persisting Effects
All three of the hidden Keepsakes bonuses will persist after you unequip them, similar to standard Keepsakes like 
Experimental Hammer and 
Transcendent Embryo.
For example, any Boon levels from Jeweled Pom remain upgraded even after switching to a different Keepsake in the next region.
Calling Card
How to Unlock Calling Card
Calling Card is given by 
Zagreus. You'll offered the option to face him in battle in 
Charon's Shop. Sign the Infernal Contract to face him.
After facing and defeating him multiple times (2), you'll receive his Calling Card Keepsake in return.
How to Use Calling Card
You'll be given the option to rarify a Boon just as if you were using a God's Keepsake like 
Barley Sheaf. The important distinction here, is that with Calling Card you can rarify up multiple Boons [2/4/6], or use 3 rarifies on the same Boon to take it all the way from Common to Heroic!
Calling Card is great in the first or second region of a run, when you are putting together the foundation of your build. You can either spread the rarify uses over many Boons, or dump them all in a couple Boons.
Time Piece
How to Unlock Time Piece
Time Piece is given by Chronos. After working through the post-credits storyline and achieving victory in the underworld once again, you'll start encountering Chronos during the night.
After running into him a few times, you'll be given the option to gift him, and he'll give you Time Piece in return.
How to Use Time Piece
When you walk up to receive any reward (Major and Minor), you'll be given the new option to PURGE the reward in exchange for Gold Crowns.
- Minor Rewards are PURGED for +200, and Major Rewards can be PURGED for more than +400 Gold.
Jeweled Pom
How to Unlock Jeweled Pom
Jeweled Pom is given to you by the reunited Hades and 
Persephone.
As with the others, you'll need multiple runs to encounter them and go through a series of dialogue before getting the chance to offer them a gift, and receive their Jeweled Pom Keepsake.
How to Use Calling Card
After equipping Jeweled Pom, you don't need to actively do anything else. When offered Boons, some will come with +Pom levels already (similar to 
Aspect of Persephone). You can take the Boons with increased ranks or not, the choice is yours.
