Elemental seepage is elemental fire, cold, thunder, or lightning, or some combination of the four. The seepage takes the form of a strange, viscous mass that pulsates and quivers with the energy of the Elemental Chaos. In general, elemental seepages are harmless, though some contain one-way, imprecise portals to the Elemental Chaos.
Effect: An elemental seepage appears as an area of at least 4 contiguous squares. A creature within 5 squares of an elemental seepage square that uses an attack power with a keyword corresponding to any of the energy types forming the seepage gains a +1 bonus to attack rolls per tier with that power.
Usage: Use this terrain to give creatures with attacks that match the seepage damage type extra hitting power. If you include only a few elite creatures whose attacks match the terrain’s damage type, it creates an interesting tactical situation where the characters are trying to keep those creatures more than 5 squares from the seepage.
Published in Manual of the Planes, page(s) 22.