Submitted to Gorgonmilk’s sources of magic community project:
The Gibbering Sea
At the end of the last age, the armies of Law and the hordes of Chaos battled in the valley pass between Ancient Kardmon and the Blasted Heaths. Thousands died, and beleagured chaos wizards unleashed the Sleepless Dust from Beyond the Pale. Reality collapsed under the eldritch weight, and the valley vanished, leaving behind a crater hundreds of miles in diameter and scores of yards in depth. In the centuries since, rain, run off from snowcapped peaks, et cetera, have filled the crater with icy water stained black by the impure earth of the crater. When the ebon waters bubble and lap on the shore, the sounds of countless voices gibbering in sorrow can be heard.
Chaotic spellcasters who survive on nothing but the Gibbering Sea’s foul waters in the surrounding wilderness for a fortnight can tap into the unholy power of the crater. (Those that don’t survive are seldom seen again, but a few wander back to civilization irrevocably insane.)
A chaotic spellcaster who has tapped into the Gibbering Sea’s power selects his spells as normal for his class. He can, however, select one spell of a level no higher than one less his maximum allowed spell level from another class’s spell lists by praying to the sea’s essence. For example, a cleric that can cast 1st- and 2nd-level spells could select a 1st-level magic-user spell, and a magic-user able to cast up to 3rd-level spells could select a 1st- or 2nd-level cleric spell.