These Nar demon binding artifacts hail from a dark time in Narfell’s past.
The idols themselves are known as the Kor’Nar Relics.
They were created by a demon binder named Tuhl Vass. So potent was the working of the magic, that he forged his own soul into its creation. It is said that in addition to his own soul, he had to fuel it’s creation with innumerable sacrifices. Furthermore, he bound the spirits of eight Tanar’ri of great might into the idols themselves, bestowing each idol with vile intelligence, its own personality, demands, and purpose.

As to their purpose, the accounts vary. Some describe the idols as purely religious, bestowing the unholy blessings of the Abyssal Lords upon the lands they touch. Other accounts speak of their ability to open gates to the gaping maw of the Abyss itself, calling forth the denizens of its infinite depths.
Yet further accounts speak of them as having to work in unison to open gateways to any place or time. Suffice it to say that they have the potential to act as planar gateways, and as a means to summon Abyssal creatures.
On their own, they are potent foci of corruption of frightening proportions, but together, they represent a serious threat. The summoning force they can exude in unison is, research suggests, strong enough to open rifts in reality, and summon forth gods in Avatar form.

There is a caveat, however. For these idols to work, they require immense amounts of vile magical energies. They need to be charged up, as it were. In the Abyss, the most potent form of energy, magical or otherwise are souls. Souls are the currency of the lower planes. This illustrates why.
Each Idol is listed below, by name, and how they may be charged. It explains that mere proximity isn’t required to charge them, but it does speed things up.

The Idols are:

K’ha Huwarn: The Idol of Bloodletting
Believed to be powered by the sacrifice of at least 3844 souls
All evidence points to this one being the one you recovered from Gromgar.

Graz’zat Va: The Idol of Domination
Believed to be powered by the sacrifice of a monarch or absolute ruler
This seems to be the idol you saw being removed from your shrine in Suzail

Glor’Mog: The Idol of Greed
Believed to be powered by the sacrifice of 31 wealthy souls
Last seen in the Empire of Narfell. What is now Damara

Tzeen’Chavek: The Idol of Evolution
Believed to be powered by the sacrifice of 31 mutated souls
Last seen in Rashemen

Grovorrg: The Idol of Gluttony
Believed to be powered by the sacrifice of 31 gorged souls
Last seen in The Great Dale

Narg’lavart: The Idol of Pestilence
Believed to to be powered by 248 souls lost to disease
Last seen in Vaasa

Fraz’zk Ta: The Idol of War
Believed to be powered by 7688 souls lost to battle
Last seen in the Empire of Narfell. What is now Damara

T’seev Esh’ka: The Idol of Vanity
Believed to be powered by 31 souls lost to suicide.
Last seen in the Great desert of Anauroch.