Offensive alchemy consists of deliberately failing to perform alchemy properly, resulting in a fireball centered on the PC's location. Unlike the Fireball spell, the resulting fireball affects the PC as well as nearby squares, so players wishing to make use of it should ensure that they have fire resistance and something to protect their equipment from damage (ring of ice or fireproof blanket). The fireball radius is random, and can be between one and three. In earlier ADOM versions, the radius was determined by willpower, like ball spells.
To perform offensive alchemy, simply combine two potions or herbs that don't match one of the given alchemy recipes. Any combination works, even though it seems odd that mixing two potions of carrot juice or water could cause a fireball and damage nearby monsters.
Since there is no limit to the number of herbs that can be harvested, alchemists may simply combine any two herbs to obtain an unlimited number of explosions - real alchemical recipes never combine two herbs.