Oblivion Gate

Overview

These gates of Oblivion are portals that are opened linking the mortal world with the planes of Oblivion. These gates are big and bright which make them visible over long distances. When the player approaches a gate the sky turns red and the gate expels fire.  The rare Bloodgrass and Harrada alchemical ingredients, usually only found inside active gates, will sometimes grow at the location.
 
The gates are constantly letting Daedric monsters out of the planes of Oblvion and into the mortal world where they attack citizens. Once the player enters the gate there two ways out: the first is just walking back through the fiery portal and the second being the destruction of the gate by removing the power source which is the Sigil Stone. 
 
In addition to some fixed quest-based Oblivion gates, they also appear at random through Cyrodiil, the frequency of their appearance changing depending on the player's progress through the Main Story, first appearing after Martin is taken to Weynon Priory, causing 25 gates to immediately open.The player is most likely to encounter the gates after finding the Mythic Dawn shrine and handing Mysterium Xarxes over to Martin, at which point there can be up to 50 random gates open simultaneously. After returning the Amulet of Kings, only a mere 20 gates will open. Once the main quest is finished, or a total of fifty random gates have been closed by removing the Sigil Stone, no more gates will open and all open gates will automatically seal.