Hellfire

Hellfire is found predominantly in the Nine Hells.

    Effect: A square of hellfire casts bright light in a 20-square radius. The ranged attacks of creatures adjacent to a hellfire square gain the fire keyword. A creature that enters a hellfire square or starts its turn there takes ongoing 5 fire damage per tier (save ends). The ongoing fire damage increases by 5 per tier with each failed saving throw (to a maximum of 20 per tier).
    Usage: This terrain obviously fits encounters in the Hells, but it also makes sense in other encounter locations where devils are present, in temples and shrines dedicated to Asmodeus, or other fiery locations. Use it to give devil worshiping cultists ranged fire attacks, and include some fire-resistant creatures with forced-movement powers that can force the characters into the hellfire. For a difficult challenge, mix in creatures with powers that apply a penalty to saving throws against ongoing fire damage, such as genasi fireblades.

Published in Manual of the Planes, page(s) 22.