The dramojh were not of the Dark

The most consistent Legacy Of The Dragon, that is of the dramojh, was body-horror. They'd maybe or maybe not use the Seeds directly, but they more used what they had already learnt from the Seeds. The dragon-scions would occasionally also pull from outer planes (the Alabast are testimony to that) but they generally stuck with Terrakal, in-house. The dramojh also pulled from the Dark, making Verdune into a land of the undead... or so says Diamond Throne.

I'll posit here that the dramojh's relationship with the Dark, like their outerplanar games, was not core to the dramojh.

Again: Mike Mearls points the way in Mystic Secrets. Here, refer to his Herald Of Annihilation - the antirunechild. This one does pull from the Dark. Back when the dramojh were still around, a Herald might arise who pulled so much that the dramojh had to put him down. But toward the end it seems even the dramojh weren't always up to such an endeavour.

Let's say this - the dramojh did what Thomas Covenant did in The Illearth War, they tore the veil between life and death. This raised the Dark, and this was done in Verdune. That land became so unhallowed that the dramojh didn't even want to touch the place anymore; thus barring them from further west. When the dramojh were eradicated, the Death King (Legacy of the Dragon) "lived" on, in Verdune's former capital.

Who was the Death King? Perhaps he was the last Herald of Annihilation - and the first to succeed. Perhaps he was a Champion Of Darkness. Perhaps he allied with the litorians in their rebellion from the Plains. Either way, it seems befitting this Morally Ambiguous setting that his main act in life, or in "life", was to oust the dramojh from Verdune... and that the Diamond Throne would rather we all not know that.