Inferno is not a new boss, since he was the original final boss for both Soul Blade/Edge and Soul Calibur I. In the first game of the series, he is simply listed as Soul Edge, which makes it seem like his entire existence is directly tied to the dark, evil sword. In my opinion, though, he is a rather lame final boss.

He appears at the very end of Arcade Mode no matter which character you choose. Even if you pick Nightmare, who already wields the same Soul Edge sword that Inferno hovers under.

The main reason he kind of sucks is that he plays just like Charade. Technically, you fight him in a single round, but it has multiple phases. Over time, he just adopts a full moveset and ends up fighting exactly like one specific character. Unless you have the difficulty set fairly high, he is usually pretty easy to defeat.

Apparently, you could actually play as Inferno in Soul Calibur II if you used an Action Replay. Personally, I get more enjoyment out of watching him be defeated, mostly for good, as the series later shifted its focus to different villains.

With his defeat, the Soul Calibur sword is finally freed and no longer corrupted by his darkness. I would have rather had the opportunity to wield the legendary blade of good against him in the final boss battle as a special surprise, instead of facing something that feels mostly nominal to the story.