Teeth of Diablo appears to be some sort of power-infused spikes, used by Triune cultists in making Dark Vessels, and subsequently Activated Vessels, but also hold additional powers of unknown potency.
Mysterious Teeth
Activated Vessel concept art, showing "Teeth of Diablo".
Artwork showing Triune Leader Blood Guard with "Teeth of Diablo" studded hammer. Remarkably similar to Berserkers.
Not much info is available but the assumption can be made that these spikes, which are infused with symbols, may be the source of some demonic power.
One illustration contains a Triune Leader Blood Guard, with a studded hammer that contains Teeth of Diablo to make it more... well... pretty? Maybe it does more damage due to the teeth. Maybe it is the teeth that allows the Blood Guard to go into a berserker state. We just don't know yet.
The only reference in literature to spikes that may refer to the Teeth of Diablo can be found in the Writings of Abd al-Hazir, more specifically the 42nd entry regarding Dark Cultists. While reading the entry you get a feeling that the entry and the illustration of a Dark Vessel and an Activated Vessels are linked. In the entry they talk about spikes being driven into some complacent victims’ back, which is clearly the case in the illustration.
But this is all just speculation, we just don't know enough to really be able to say anything with any degree of confidence.