Tommen II sailed the great fleet of House Lannister to Valyria after the Doom, to try and plunder wealth and riches. It was a quest from which he never returned. The ancestral sword of the Lannisters, Brightroar, was lost with him.
He is referred to in the chronicle titled The Glory of Volantis, and it is said that he was welcomed into the city, and that the triarchs showered him with gifts. He supposedly said that half of what he plundered he would give to the triarchs as thanks for their generosity.
The next year, one of the triarchs, Marqelo Tagaros, dispatched a squadron towards Valyria to look for signs of Tommen's fleet, but nothing was found.