Half-Moon Henge and Ruins

An ancient holy place. Who built this monument and for what purpose is mostly unknown. The civilization which belonged to it no longer offers any clues as to its origin, but one thing is known: the air within the henge is thick and sour with magic. Portals to other planes occasionally flicker, but these are rare and unstable; they are either a side effect of whatever brought ruin to these lands, or they no longer function as intended.