Demon Trophy was a Hardmode consumable dropped by the Wall of Flesh in all difficulty modes. It boosted enemy spawn rates in the current world by 25% and was not reversible once used. It could only be consumed once.
The effect of the Demon Trophy would stack with spawn rate modifiers such as
Water Candles, Battle Potions, Revengeance Mode, etc. Once the Demon Trophy had been used in the world, the Wall of Flesh would no longer drop it and it would also no longer be contained in its Treasure Bag.
It could help make grinding for rare items more efficient due to increasing the spawn rate of enemies.
Though the Demon Trophy effect was permanent, buffs that reduced enemy spawn rate such as Zen Potion would alleviate much of its effect when increased spawn rates were not desired.
In previous versions of the mod, the trophy toggled on or off Prepare to Die mode instead.
In the mod's source files, the trophy is referred to as 'MLGRune' in reference to its previous function.