This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

adj. So difficult or taxing as to make one exhausted; backbreaking.

adj. characterized by effort to the point of exhaustion; especially physical effort

"Instead of giving their chapters proper titles, these worthy gentlemen simply slap a character’s name at the top of a fresh page, followed by his or her inmost thoughts and experiences in gruelling detail."

effortful   

backbreaking    exhausting