The Oath of the Unseen (Vows) allows you to customize your endgame challenge level in Hades 2. Here is a list of all the Vows. Each one has ranks, effects, and number of Fear.
Hades 2Oath of the Unseen (Vows) - Hades 2 Wiki

Vow of Debt
All items that cost Gold are +40%/+80% more expensive.
Upon this night I shall have naught to spare, that Fear may spread.

Vow of Denial
After you choose a Boon, 2 unpicked blessings shall not appear again this night.
Upon this night shall blessings be withheld, that Fear may spread.

Vow of Fangs
Most foes with Armor also have 1/2 random Perk(s).
Upon this night shall newfound terrors manifest, that Fear may spread.

Vow of Forfeit
The first 1 Boon(s) in each Region become Ashes instead.
Upon this night shall favors turn to tears, that Fear may spread.

Vow of Frenzy
All foes are +20%/+40% faster.
Upon this night I shall be set upon relentlessly, that Fear may spread.

Vow of Grit
All foes have +10%/+20%/+30% Life.
Upon this night my foes shall not fall readily, that Fear may spread.

Vow of Hordes
Most encounters have +20%/+40%/+60% more foes.
Upon this night I shall be overwhelmed, that Fear may spread.

Vow of Hubris
After you choose a Boon, Prime 3/6 for each Rarity greater than Common.
Upon this night shall my pride of gods fall short, that Fear may spread.

Vow of Menace
Most foes have a 10%/25% chance to be from the next Region (if there is one.)
Upon this night shall distant dangers come, that Fear may spread.

Vow of Pain
All foes deal +20%/+60%/+100% damage.
Upon this night I writhe, that Fear may spread.

Vow of Return
Most slain foes have a +25%/+50% chance to become a Revenant.
Upon this night my foes shall rise again, that Fear may spread.

Vow of Rivals
The Guardians of the first 1/2/3/4 Region(s) shall be stronger in various ways.
Upon this night shall adversaries rise, that Fear may spread.

Vow of Scars
Any healing effects are 75%/50%/0% effective.
Upon this night my wounds shall never heal, that Fear may spread.

Vow of Shadow
All Encounters with Wardens contain no fewer than 1 Shadow Servant(s).
Upon this night shall shadows multiply, that Fear may spread.

Vow of Time
You have 09:00/07:00/05:00 to fight through each Region (or else...).
Upon this night there shall be not a moment's peace, that Fear may spread.

Vow of Void
You have access to 60%/40%/20%/0% of your Grasp for your Arcana.
Upon this night shall my beliefs be gone, that Fear may spread.

Vow of Wards
All foes have 1/2 Armor.
Upon this night I strike to no avail, that Fear may spread.