Warnings and Spoilers for
The Goblin Twins
The sexual assault is between main character A'bbni and antagonist Hi'jan. It is described graphically but non-erotically at the end of Chapter 2, and the characters refer to it throughout the book. It involves A'bbni being gagged and bent over a table while being forcibly restrained. His father's decapitated head is also on the table next to him, and his brother, Shi'chen, is forced to watch the assault. If you wish to skip this section, you should stop when En'shea says, "But we do not want to," and skip to the beginning of Chapter 3. A'bbni suffers from PTSD throughout the book as he tries to process this trauma, including an anxiety attack while trying to have sex with Lai, his love interest. Shi'chen also suffers from PTSD, though not to the same extent as A'bbni.
There are several instances of physical, emotional, and psychological abuse of main characters by the antagonists against Shi'chen and A'bbni, mainly in Chapter 2 and toward the end of the book. This includes threats of physical and sexual violence, being forcibly held in place, being grabbed and pulled by the hair, and being threatened with weapons.
There is graphic violence throughout, both on and off page, including a side character death after an accident.