Crispus was the oldest son of Constantine. He was made Caesar in about 316 and groomed along the military and political ways to be expected of a future emperor.
However, his career was cut short in 326 when his stepmother Fausta accused him of attempted rape. Her story must have sounded like an unspeakable atrocity for Constantine had his son put to death at once.
Some time later it was somehow found out that she’d made up the entire story and Constantine had her literally boiled to death.
Fausta’s motive most likely had to do with removing Crispus from the line of succession to make way for her own sons. This much she was successful at since they did indeed become emperors some ten years afterwards.