In TW1, the tooltip for Igni's talent levels says "Damage depends on sign intensity" with no corresponding mention of whether the sign intensity increases as you level it. As other signs with explicitly mention their intensity increases, this leaves the player to possibly infer that the only benefit to leveling is to get a wider cast (making level 5 a questionable investment as it only goes from 315 degrees to 360 degrees for an increase in cost from 20 to 25 Endurance and a gold talent). My testing shows it does increase sign intensity, going from ~210 to ~270 damage on garkains the swamp cematary unbuffed.
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