While "the dwarves of Dum have captured and bound Cor-Dum" does seem fitting, it does not seem to be sufficient to explain everything we've seen here. And I strongly suspect Athel Loren is involved somehow.
• Athel Loren are the ancestral foes of Cor-Dum, and have spent centuries trying to put him down permanently.
• We know that prior to the great Chaos Incursion, the high elves were in hot pursuit of Cor-Dum across the old world - and that Finubar, who was in charge at the time, is far more open to cooperation with the dwarves than the usual.
I can imagine a sequence of events where Karag Dum agreed to help the wood and high elves seal away Cor-Dum, or something of the like, only to bust out their pet sealed evil in a can as their final contingency when it looked like all other options had failed.
The shame of Dum would thus be threefold - that they had worked with the elves to bind a respawning chaos puppet, that they hid this from the rest of the Karaz Ankor, and that they resorted to actively deploying their bound evil to keep themselves alive.
I suspect that the forest we're seeing was created through wood elf magic, either through direct cooperation with said wood elves or through the runemasters redirecting wood elf magic that had previously been used to seal Cor-Dum.