Because Agni is not a sura and cannot change into sura form, he can only achieve some relatively minor effects; it also puts him into a special buff state. As long as the buff is maintained, all living beings within range are unable to regenerate. When a target takes damage beyond the range of the buff, the regeneration speed from that damage will be reduced.
While in this state, Agni appears darker than normal, and all of his transcendentals look black. Furthermore, he is unable to use White Assimilation, nor any transcendental skills that suras cannot use (such as Paradisial Flare and Fire Assimilation). It is different from White Assimilation, but his temper becomes quite violent and he does not joke around like usual. (But compared to Taksaka, the owner of this transcendental skill, he still has a very nice personality.)
Agni learned this skill quite a long time ago via Sword of Hellfire, but this skill can only be used in sura form. It was only after learning Taksaka's Extermination, a sura-form offensive transcendental skill, that he discovered that he was able to use Vritra clan sura-form transcendentals...but Agni has yet to tell the other gods that he is able to use them.
In Kalibloom, as Agni tries to calm Gandharva after his rampage on the city, he uses Sword of Hellfire to summon the sword that allows him to use Extermination, which in turn allows him to use Fathomless Fire so they can speak without eavesdroppers.