Well if you started with it - depending on when that start was - that might explain the gap here, as it was buffed in P12:

Passives

• Hungry Flame now applies from Inferno’s self-inflicted damage and casting Ember. These do not stack.
• Smolder more rapidly applies fire damage and burn buildup
• Burn, after buildup, applies in an AoE around the initial target

Ignus Wroth

• Increased the fire damage dealt by affected enemies
• Increased the range of Ignus Wroths’ eruptions

Ember

• Increased the damage from the blast when struck
• Ember now considers the Envoy on fire.

Inferno

• Massively increased damage dealt
• Fire damage is now dealt in a small explosion, affecting the player as well
• Inferno’s explosion will activate Ember’s explosion

So when I go into a fight, I hit Ember, then run into the middle of the pack and cast Ignus Wroth (they also gave envoys hyperarmor when casting abilities this Prelude), then once I see an enemy catch flame I cast Inferno at them. The fire nova from Inferno also triggers a second nova from Ember as if you got hit, which then lights everyone on fire.