The problem arises when he is making attacks while in "stealth" or casting while in "stealth." This isn't a legal action, it's a bug; launching an attack breaks stealth whether or not it misses, and casting breaks stealth at the start of the cast, not the end of the cast. It can be safely concluded that he's not in stealth, but his model is failing to render for one reason or another.
A lot of enemies who are said to be "great sneaks" are not being detected by well-invested detectors, they're just killing people while invisible as "Someone" floods the combat log. This is a bug we have a lot of evidence of occurring semi-regularly (for some, they're unlucky enough it happens every time they go to these places where it's happening). It's coming up as frequently as tileset-based AOE spell issues and we have to alter our play or flee to get around it.
After closing my game completely and re-launching it 4 times in a single Malthor battle, I can't recommend anyone risk their party wiping to random damage from an untargetable enemy (or group of enemies) by relogging or spamming detection buffs. These enemies are not in stealth. Pulling him out of the cave is the only sure bet. I recommend the same thing with the mummies if you notice it happening before it's too late, because by going back inside, you risk these enemies finding you and making all their attack rolls unopposed.
