
Surtr is the unique Warchief of the Dragon clan and he has one of the highest attack stats of any warchief the draconic frenzy lore also doubles his attack when fighting without other military allies which can help in the early game. He has a chance to enslave any enemy clan members he personally defeats as a thrall rather than killing them. This chance gets lower as your number of thralls increases.
In mission 1 of Cross of Vidar Surtr comes from somewhere across the nine realms next to bifrost and kills Hvedrung and warns the others if they do not follow them he will make sure they die splitting up the group.
In mission 10 it seems he will be the final boss before fenrir spawns in and surtr dies thanks to Egil taking action
Though he is mentioned in other places through the story these are his main appearances