For a general description of rifts and a list of discovered rifts, see this page.
The Aecris Rift to the Shadowfell, named for the original name of the keep erected to secure it, is located in the mountains north of Winterhaven, somewhat near the ruins of Bael Turath. Following the fall of the keep, the building became known as Shadowfell Keep; however, the original name for the Rift was kept to avoid confusion, there being multiple rifts leading to the Shadowfell.
Originally sealed by a squad of battle-clerics following its discovery, the rift was emblazoned with runes designed to stem the corruptive influence of the rift, and Sir Keegan designated to act as commander of the hastily-erected fortress. Eventually, however, the dark power of the rift eroded the divine consecrations placed around the site, and the rift’s influence drove Keegan mad, leading to the aforementioned Fall.
Some time after this, a cultist of Tiamat named Kalarel, following divine instructions, located the rift, which he strengthened with dark rituals meant to deepen the rift’s connection to the Shadowfell goddess’ agents – and possibly Tiamat herself – through. Some of Kalarel’s observations are recorded in his journal.
The Aecris rift was sealed by the 7th Eye of the Dragon, who slew Kalarel and resanctified the temple using holy relics found in Sir Keegan’s crypt, after being dispatched by the Hand of the Archon of the Dragon’s Eye, Anabran Cane, to investigate attacks by goblinoids near Winterhaven.