Complex sentence: He and his family start to receive threatening phone calls from Stark, which Pangborn thinks it's Thad trying to pull a prank on them, until Stark begins to describe how he is going to kill Thad's family, disturbing Pangborn.
Split into simpler sentences: He and his family start to receive threatening phone calls from Stark. Pangborn initially suspects the phone calls are a prank by Thad until Stark begins to describe how he is going to kill Thad's family, disturbing Pangborn.