Complex sentence: Unbeknownst to them, Axel's reckless gambling on basketball games has left him with a huge debt to his bookie Hips, who likes the professor personally but threatens him with grave consequences if he doesn't pay up.
Split into simpler sentences: Unbeknownst to them, Axel's reckless gambling on basketball games has left him with a huge debt. His bookie Hips likes the professor personally but threatens grave consequences if he doesn't pay up.