I'll quote the relevant bits.
For example, Dragon #341 has an article I authored which provides two new vestiges (Kas the Bloody Handed and Primus, the One and the Prime), each with a suite of new powers.
Vestiges are spirits detached from the multiverse. They exist not on any plane or in any place anyone can reach. Their only window into reality comes from being summoned by binders making pacts with them. That's why they do it--to get a glimpse outside the otherworldly prison in which they somehow reside. Binders allow them to peer through the bars. These spirits might once have been angels, devils, gods, or mortals--and examples of all of those exist in the book--but existing separate from reality has twisted them and driven the mad. Vestiges aren't really good or evil. They lend their abilities to anyone with the power to reach them, granting power just to hear, see, and feel again.
Primus gone from the Multiverse? Makes you wonder what happened to the Modrons...
unless, of course, it's a previous Primus that got stuck as a vestige.
Gotta read the magazine before being a sensationalist, big man.
According to the new Dragon articke, the Primus slain by Tenebruous has returned as a vestige, a strange revenge-mad cretaure of law, marked by its own death at Tenebruous's hands. The new Primus, the real modron leader, has called his modrons back to Regulus and hsut the doors until the slight chaos int he heirarchy is resolved.