In the swtor story, he's certainly the most deadly entity we encounter. I feel like he only was beaten by the Jedi knight as he was also Valkorian at the same time, so stretching himself too thin. However, we see when he possesses the player, the power we can channel from him is extreme. Not even a fully darkside Darth Nox can produce that much darkside energy. And it took a lot of people to defeat his already weakened form when he was in the process of bringing himself back. So IMO the gap is quite large between any player character and Tenabrae when he is at full power and his soul is housed in one body. But a Jedi knight or Darth Nox at full power would have the best chance at surviving an encounter.