The built-in :crypto library includes the :ed25519 curve (as long as the installed openssl lib supports it) as seen by:

iex> :crypto.ec_curves()
[:secp160k1, :secp160r1, :secp160r2, :secp192k1, :secp224k1, :secp224r1,
 :secp256k1, :secp384r1, :secp521r1, :secp192r1, :prime192v1, :prime192v2,
 :prime192v3, :prime239v1, :prime239v2, :prime239v3, :secp256r1, :prime256v1,
 :wtls7, :wtls9, :wtls12, :brainpoolP160r1, :brainpoolP160t1, :brainpoolP192r1,
 :brainpoolP192t1, :brainpoolP224r1, :brainpoolP224t1, :brainpoolP256r1,
 :brainpoolP256t1, :brainpoolP320r1, :brainpoolP320t1, :brainpoolP384r1,
 :brainpoolP384t1, :brainpoolP512r1, :brainpoolP512t1, :sect163k1, :sect163r1,
 :sect163r2, :sect193r1, :sect193r2, :sect233k1, :sect233r1, :sect239k1,
 :sect283k1, :sect283r1, :sect409k1, :sect409r1, :sect571k1, :sect571r1,
 :c2pnb163v1, ...]
iex> :crypto.ec_curves |> Enum.filter(fn a -> Atom.to_string(a) |> String.contains?("25519") end)
[:ed25519]