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.