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.
Student or Learner
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.