So a number of the Old Lords were killed prior to the RoD, but we don't know the exact figure. But this "high company" included some of Kevin's "truest friends and strongest allies", so I would have to guess they were among the most powerful Lords on his Council. So, yes, my understanding is that Kevin was driven over the edge into despair because he blamed himself for having sent his fellow Lords into ambush.For here at Treacher's Gorge, Lord Foul the Despiser revealed his true self to High Lord Kevin. Here was struck the first blow of the open war which ended in the Ritual of Desecration.
Before that time, Kevin Landwaster doubted Lord Foul without knowing why--for the Despiser had enacted no ill which Kevin could discover--and he showed trust for Lord Foul out of shame for his unworthy doubt. Then, through the Despiser's plotting, a message came to the Council of Lords from the Demondim in Mount Thunder. The message asked the Lords to come to the Demondim loreworks...to meet with the loremasters, who claimed knowledge of a secret power.
Clearly, Lord Foul intended for Kevin to go to Mount Thunder. But the High Lord doubted, and did not go. Then he was ashamed of his doubt, and sent in his stead some of his truest friends and strongest allies. So a high company of the Old Lords rafted as was their wont down the Soulsease through Andelain to Mount Thunder. And here, in the roar and spray and ill of Treacher's Gorge, they were ambushed by ur-viles. They were slaughtered, and their bodies sent to the abyss of the mountain. Then marched armies...out of the catacombs, and the Land was plunged all unready into war.
Which brings up a question: if these Lords were indeed very mighty, why couldn't they call up some wicked Earthpower to defend themselves, even if they were ambushed and unarmed? Must have been one heck of an ambush to have been able to kill Lords of such power. And what of those Bloodguard who I assume were with them? Couldn't the Bloodguard have staved off the ur-viles' attack long enough for the Lords to regroup and fight?
As for the RoD itself, it's curious that SRD provides little detail about such a pivotal and cataclysmic event in the Land's history. Maybe it was his intention to describe the RoD only briefly, in order to allow our imaginations to conjure up our own visions of hell on earth? Or maybe SRD is saying: Look, there's not much to describe. It's (mostly) the end of all things. Deal with it.