Deepshimmer, often called faerzress, was a magical radiation only found in the Deepways, a remnant of ancient Spellcraft that helped shape the tunnels themselves. Areas of high deepshimmer concentration would manifest physical signals, ranging from subtle shifts in temperature to visibly glowing rock formations. Many plants and fungi in the deepways found ways to live on a diet of deepshimmer, such as Bluecap.
The word "faerzress" was composed of the Dark Elf words faer (magic) and Z'ress (to hold dominance or to remain in force), which together mean "magic that remains". Drow sought out areas with strong faerzress to build their cities and outposts in order to keep others from scrying their doings or teleporting inside the city. Drow also used faerzress to create magical artifacts and sustenance.
Deepshimmer could affect magic and spellcasting in unpredictable ways, in some ways like Wild Magic. It was known to mainly affect the divination and conjuration Schools of Magic, making teleporting by magic in the Deepways a risky effort. It was possible to enact rituals to alter the concentration of deepshimmer in an area or to affect a pocket of it, although such spells often attracted unwanted attention from denizens of the Deepways naturally attuned to the shimmer.