sempiternal (adj.)

"eternal and unchanging, perpetual, everlasting," early 15c., from Old French sempiternel "eternal, everlasting" (13c.) or directly from Medieval Latin sempiternalis, from Latin sempiternus "everlasting, perpetual, continual," from semper "always, ever" (see semper-). Compare aeternus from aevum (for which see eternal). Related: Sempiternally.

Trawthe is immortalle, immutable, and sempiternalle.

[Higden's "Polychronicon," 15c. translation]