This was a torus-shaped metallic contraption designed by Doctor Doom. Doom could open it to a semicircular shape and hold it in a hand while attacking, then close it on an enemy's neck. The collar then would reduce its circumference whenever the victim uses its power, thus threatening to strangle the victim. Doom used it once against Blastaar in the Negative Zone.
This item was seen only once and a number of points are still unclear about it - including even its full name, Doom simply called it "the collar". We also do not know the item's limitations, such as whether it works on any kind of superpowers (even artificial ones like Doctor Doom's Armor?) or even whether it works outside the Negative Zone.