Mether's Breath mechanics below are community-reported from early launch guides. The unlock threshold and Ova numbers vary slightly between sources, so treat precise figures as unconfirmed until verified in-game.
Fast travel in Mortal Shell II is called Mether's Breath. It is not available from the start: you unlock it by collecting Ova and returning them to Marrow Keep.
Once unlocked, you can teleport directly between any Beacons you have discovered, and also use Mether's Breath to cleanse corruption walls blocking certain paths.
How to unlock Mether's Breath
Collect Ova by cleansing Beacons out in the world, then return to Marrow Keep and deposit them at the Gloom Siphon (also called the Ovum Siphon) in the Ova Nursery. After you contribute enough Ova, Mether's Breath unlocks automatically.
You start the open world with 3 Ova. The most commonly reported total needed is around 7-8, so you generally need to collect 4-5 more from cleansing Beacons. The exact number varies slightly between guides, so treat the precise threshold as unconfirmed.
Fast Ova sources
The fastest early Ova come from Beacons. Regular Beacons grant 1 Ova each, while Beacons next to Corrupted Gates grant 3 each. Clearing a full Corrupt Gate and its final boss can reward roughly 25-26 Ova.
Mushroom Village Beacon gives 1 Ova, and Mushroom Village Gate Beacon gives 3.
How to use Mether's Breath
Interact with any discovered Beacon and choose the Mether's Breath option to fast travel between Beacons. It also lets you cleanse black tendril corruption walls, and return to Marrow Keep from your inventory.
Note that fast travel is temporarily unavailable during Gland Runs, when you must physically carry a Gland back to Marrow Keep.