In an unusual scene in the bustling US Congress, Caesar testified before the Senate Foreign Relations disguised under an oversized blue hoodie with the audience and media ordered to turn off all cameras and phones to ensure his safety.
Caesar said that, despite the personal risks he has taken, he had not achieved his goal of ending the abuse.
"Killing has increased in the same places and in the same ways and at the hands of the very same criminals," he said.
"And the reason simply is that the Assad regime considered the inaction of the international community and the mere statements of condemnation as a green light for him to continue his crimes against the Syrian people."
A previous appearance before the US Congress by Caesar in 2014 led lawmakers to draft a law named after him that would impose financial restrictions on Syria, including blocking reconstruction aid, until perpetrators of atrocities are brought to justice.
The act, which also penalizes Russian and other companies that work with Assad, was finally signed into law in December by President Donald Trump after years of fighting inside Congress.
Caesar hailed the act but called for lawmakers to ensure it was enforced.
"The Caesar Law has become the only ray of hope for the Syrian people in the absence of any military or political solution," he said.
"This law is a powerful message to all who support the Assad regime that the time for accountability and justice is coming and that no matter how long oppression lasts."
The testimony comes amid a fresh humanitarian crisis in Syria as Assad, backed by Russia and Iran, pushes to retake the Idlib region from rebels who include Turkish-backed Islamists.
The bloody assault has pushed close to a million civilians northward toward Turkey's border.