Mervyn Peake invented a lot of words in his writing. However, a very quick Google search for the word found many hits, like this one.
The passions, no greater than candle flames, flickered in Time’s yawn, for Gormenghast, huge and adumbrate, out-crumbles all.
Behind him the long, adumbrate Hall of the Bright Carvings glimmered beneath the suspended chandeliers.
(M. Peake; Titus Groan)
Is it an adjective here? None of my dictionaries lists it as such.