Alieu Kosiah was a rebel commander fighting against former President Charles Taylor's army in the 1990s.
In 2021 he was sentenced to 20 years in prison in Switzerland's first-ever war crimes trial.
At his appeals trial, opening at Switzerland's Federal Criminal Court, that sentence could be increased.
The indictment has been expanded to include crimes against humanity.
It's the first time these charges are being brought in a Swiss trial.
Lawyers say that if the Swiss court accepts that crimes against humanity were committed, Kosiah's sentence could be increased to life imprisonment.
Thousands of people were mutilated and raped during Liberia's back-to-back wars between 1989 and 2003 in clashes that involved drugged fighters and child soldiers conscripted by warlords.
But unlike neighboring Sierra Leone, which had its own civil war at the same time and later held war crimes trials, no prosecutions have taken place in Liberia.
Kosiah was arrested in Switzerland, where he had been living, in 2014.
Swiss law allows for prosecution for serious crimes committed anywhere under the principle of universal jurisdiction.
Kosiah's defense has denied the charges and previously argued that he was a minor when first recruited in the conflict, and was not present when the crimes were allegedly committed.
Seven of the victims, including a woman who was raped and a man who alleged that he witnessed the defendant eating slices of a man's heart, are set to appear.
They have all asked for anonymity for fear of reprisals.
Some warlords still hold influential positions in Liberia.
But momentum is growing outside the West African country for justice for war-time atrocities.
In November a former Liberian rebel commander was sentenced to life imprisonment by a French court.
On Tuesday (January 10) a Finnish appeals court began hearing a Liberia war crimes case.
The Swiss appeal trial is set to last until February 3 and a final judgment is due later this year.