Ah, but in the Five Songs of King Wulfharth he was with them inside Red Mountain. According to Wulfharth it was Lorkhan, Wulfharth, and Voryn Dagoth allied against Nerevar, Dumac, and Alandro Sul. Wulfharth's take on the battle that follows:
Wulfharth cannot hit Sul, but Sul delivers a mortal blow to Wulfharth as Wulfharth shouts Sul blind. Voryn Dagoth slays Dumac (?) but not before Sunder (presumably wielded by Dumac, Wulfharth makes no mention of Kagrenac) is able to strike the Heart making it vulnerable. Dumac's death enrages Nerevar and causes him to turn away from Lorkhan and kill Voryn Dagoth. While his back is turned Lorkhan lands a mortal blow on Nerevar. Nerevar, dying but not yet dead, uses Keening to cut out the Heart.
I'm not trying to be a jerk or deliberately dense about this, but this cannot be the way the events transpired which is why I continue to say that Wulharth can't be trusted.
Wulfharth cannot hit Sul, but Sul delivers a mortal blow to Wulfharth as Wulfharth shouts Sul blind. Voryn Dagoth slays Dumac (?) but not before Sunder (presumably wielded by Dumac, Wulfharth makes no mention of Kagrenac) is able to strike the Heart making it vulnerable. Dumac's death enrages Nerevar and causes him to turn away from Lorkhan and kill Voryn Dagoth. While his back is turned Lorkhan lands a mortal blow on Nerevar. Nerevar, dying but not yet dead, uses Keening to cut out the Heart.
I'm not trying to be a jerk or deliberately dense about this, but this cannot be the way the events transpired which is why I continue to say that Wulharth can't be trusted.
But that version includes Wulfarth dying under the Red Moutain. With every other account placing the chimers as holding the Heart and the tools in the end, it means the nords had no first hand witness available, especially after their army got routed and butchered in the mess. Which means the Song is utter rubbish as far as what happened inside the Red Mountain is concerned. But for what happened out, the mobilisation, the tip from someone in House Dagoth, there are live witnesses, which makes it more likely to be true.
One can also note that 'devil' is a generic term used by he nords for the chimers. It has no relation with the 'Devil' term used by the Temple about Dagoth Ur, which means the 'Devil of Dagoth' is not necessarily Voryn. It might even not have been a genuine house Dagoth member, as I wouldn't trust the nords to tell a chimer faction from another. Though the mention of dwemers centurions (stolen men of brass) hints that it was the case. House Dagoth had afterall the closest relation wit the dwemer and possibly access to some of their machines.