Flames of Phlegethos, sometimes known as brimstone blast, is a moderately powerful warlock invocation learned by young or inexperienced warlocks, particularly those who forge pacts with the powers of the Nine Hells. When flames of Phlegethos is cast, the warlock casting it summons from the plane of Phlegethos burning streams of liquid fire, which he bathes a target of his or her choice within fifty feet, causing them immense pain and burning at their flesh. Even if the creature survives the initial onslaught, the flames continue to burn, causing continuous pain to the warlock's victim. The power of flames of Phlegethos is largely dependent on the constitution of the warlock casting it and the spell is so powerful that it can only be cast once per day.

Prior to the Spellplague, flames of Phlegethos was known primarily as brimstone blast and was castable at-will, like all warlock invocations. Flames of Phlegethos was also slightly more powerful, but at the cost of being too much for inexperienced warlocks to master.