Mae rhyw hud yng Nghwm Derwyddon - cwm sy'n llawn o hanes a chwedlau. A chyfrinachau. Mae'r Dybyl-Bybyls yn cynnal "gwylnos" i'w tad yn Nant-y-Fagddu ac yn gofyn i Cledwyn wneud un gymwynas olaf, "bwythig, bwythig", a Rhen Crad. Ond mae Sbanish, Bic a Drwgi hefyd ar eu ffordd i fyny'r cwm - sy'n cymhlethu pethau ychydig. Ond nid dyna'r unig huddyg yn y potas. Comedi dywyll ac afreolus yw "Madarch" - ail nofel Dewi Prysor a dilyniant i'r nofel boblogaidd "Brithyll".
Dewi Prysor Williams, or Dewi Prysor as he is more commonly known, is a Welsh author and musician. He was born in Dolgellau in 1967 and raised in Cwm Prysor, near Trawsfynydd. He lives in Llan Ffestiniog with his wife and three sons.
In 1992, Prysor was accused of 'conspiring to cause explosions' as part of the Meibion Glyndŵr arson campaign. He spent 14 months in Walton Prispon, Liverpool, while awaiting a trial. He was found not guilty. He subsequently wrote a drama based on this experience called DW2416, which was performed 25 times on tour by Llwyfan Gogledd Cymru/North Wales Stage.