Thalamus is the key to surviving Hades 2's most punishing region, but it's tricky to find compared to the other crafting materials.
To find Thalamus, you need to play far enough into the game to access Chaos gates so you can descend into the pitch-black domain of creation—which you might recognize from the first
Hades. You also need to have a Silver Shovel for digging up seeds in mounds of dirt.
Right now, Thalamus only has one use in Hades 2, but it's essential to keeping you alive in the late game. And it's possible it'll have other purposes down the line as Hades 2 evolves in early access.
Where to find Thalamus in Hades 2
Thalamus must be grown from Origin Seeds in Hades 2. Origin Seeds are primarily found in the realm of Chaos, which you can enter via Chaos Gates, the circular holes in the ground with black smoke rising out of them. You have to take damage to enter them unless you force a gate to appear by paying gold at a Well of Charon beforehand.
Chaos Gates should start appearing after you've beaten the first boss at least once and have spent [[link]] a few hours going on runs in Erebus.
In the realm of Chaos, Origin Seeds can be dug up with the Silver Shovel by interacting with mounds of dirt, and then planted in The Crossroads. The "Flourishing Soil" Incantation is required to unlock [[link]] plots of soil to plant them in.
Origin Seeds will grow into Thalamus after seven completed stages during a run, or by waiting in the hot [[link]] springs or in Taverna (if you've unlocked them). When harvested, you receive two Thalamus. Alternatively, Chaos, the character, will give you an Origin Seed after your first trip to the surface.
How 
to use Thalamus in Hades 2
Thalamus is one of the crafting materials you need to survive the periodic damage you take on the surface. You need two Thalamus to finish the "Unraveling a Fateful Bond" Incantation at the cauldron in The Crossroads.
Otherwise, Thalamus doesn't have any other uses in Hades 2 at the moment. The item description says it's "mostly used with incantations involving birthright and creation," but only the incantation uses it so far. I'd keep some on you just in case Supergiant Games adds other crafting recipes in the future.