The Ordinary Chaos God Ending (OCG) is a special ending in ADOM that requires the player to overcome a number of hurdles beyond simply Closing the Chaos Gate. The Ordinary Chaos God Ending is more forgiving than an Ultra ending in terms of the requirements to find several rare items--notably the PC is not required to save Khelavaster; however, in practice, many players consider this the most difficult ending to achieve, and it is rarely attempted.
In order to achieve the Ordinary Chaos God, the player must complete Gaab'Baay's Quests to acquire both the Medal of Chaos, as well as the Crown of Chaos from the Ancient Stone Circle. In addition, the player must make a harrowing journey down to the SIL, a level located at where ID:66 would be in the Infinite Dungeon - the first time the player descends from ID:65, the SIL is reached; it must not be left or the PC will never find it again.
There, the PC will have to face MaLaKaI, an extraordinarily powerful chaos knight--fighting with spells, missiles, or under cover of darkness are essential: MaLaKaI takes approximately 8 attacks per turn, capable of inflicting well over 100 damage each hit. He can easily kill the most experienced of PCs in a fair fight in melee. MaLaKaI is standing on the Sceptre of Chaos, not actually carrying it, so in principle it possible to recover the sceptre without fighting him. Simply lure him away a few squares and teleport to the sceptre, then teleport to the stairs and leave. The Sceptre of Chaos corrupts the PC whilst it is carried or equipped, so the journey out of the Infinite Dungeon is a highly corrupting experience. Corruption can be limited by copious use of the Magic Map and Teleportation spells.
Prior to entering the Chaos Gate, the PC must also be a crowned champion of chaos, extremely corrupted (about 16 or 17 corruptions), and have an alignment of C-. If the PC enterings the Chaos Gate on D:50 with all of these conditions met, while wearing the Medal of Chaos, Crown of Chaos, and wielding the Sceptre of Chaos, they will be transported to the chaos plane to face Andor Drakon. Killing Andor Drakon will result in a victory. Note that the PC is not required to be wearing any of the Chaos Trinity once they have entered the chaos plane--indeed, once through the gate, it is best to abandon these items immediately to rid the PC of their corrupting (and dooming) influence.
The Ordinary Chaos God Ending is made challenging in three respects: first, it requires the difficult dive through the Infinite Dungeon to recover the Sceptre of Chaos. By I:66, many of the monsters in the Infinite Dungeon are quite dangerous, and unprepared PCs may face untimely deaths, although this journey can be delayed until the PC is as prepared as possible. Second, due to the highly chaotic nature of many acts required--crowning chaotic, carrying the chaos trinity, etc.--the PC is likely to spend large portions of the game with numerous potentially harmful corruptions. Third, the PC is lacking major resources available to all PCs attempting an ultra ending. The first are the six scrolls of chaos resistance awarded for saving Khelavaster (in principle, an OCG character could save Khelavaster, but players who save him rarely attempt this ending). These scrolls amount to a removal of around nine corruptions, which buys the player hundreds of extra turns of combat with Andor Drakon on the chaos plane. The second is that the player is without the most powerful weapon in the game, the Trident of the Red Rooster. Thus prospective OCGs will need to find a suitable alternative capable of doing enough damage to slay Andor Drakon without this resource available.
Note that unlike ultra endings, the OCG does not require the PC to have reached experience level 45, as killing Filk is only necessary to obtain the Trident of the Red Rooster. This is particularly nice for trolls, who rarely reach this level of experience without excessive scumming.
For players interested in increasing their final score, the ordinary chaos god ending is not recommended; a PC who manages to close the Chaos Gate and leave the chain will receive approximately 4 times as many points in their final score as an otherwise equivalent OCG. It is unknown if this was fixed in 1.2.0; though it is probably related to how final scores for ultra endings were previously calculated, which was reported to be an oversight.
Alternate Method Edit
Technically, the only requirement for completing an OCG ending is killing Andor Drakon while chaotic. This means that the ending can also be achieved by "failed" ultra ending hopefuls. In this case, the PC may successfully enter the gate with the Medal of Chaos, Crown of Chaos, and Trident of the Red Rooster, change their alignment to chaotic (if necessary), and then not be wielding the trident on the turn Andor Drakon dies. Although an Ultimate Chaos God Ending gives a significantly higher score, this is a means to complete an OCG ending without retrieving the Sceptre of Chaos.