Dark influence is a term used for anything related to the underworld. Specifically the deeper regions where Demons rage against the chains that keep them bound.
The last known incident involving dark influence was when Arch Visor Marrien used a tomb so powerful that she could finally defeat the hordes of enemies that had been holding her kingdom under siege for eight months. She did however understand what this would mean, and as such was a last resort. She wiped out the enemy in less than three hours, burning them away with fel-flame so violent that it scourged the land and turned it to glass.
Then, in victory – the dark voices in her head convinced her to slaughter her people and raise her kingdom to the ground.
And she did, with glee.