DIRECTORS CUT in Sins of Sinister directly stated by the writer.
"Devil Advocacy (From Journey of Mystery- Its basically a meeting place for all head demons in MU. Any decent demon will be. Mephisto, etc"
He stated refer below from Journey into Mystery about the Council
It is shown Adversary arguing with another demon Darklove for satans chairs in Journey into Mystery 627. Wait there's more.
The Devil's Advocacy is the meeting place for the head demons in Marvel Universe (the multiverse) and wait there is Adversary with his peers close to the chair of satan in Sins of Sinister? Unless you want to argue someone like Dormammu is from 616 and a universal reboot affected him too? Along with other demons from different dimensions too? Cable and X-Force stated he is pan-dimensional. Which means he exists as one version in infinite realities.
Infernal Parliament is literally the Devil's Advocacy referring from Journey into Mystery PER the writer
The intro for Resurrection of Magneto "FALLS THE SHADOW! In the cold dark rooms beyond death, there are terrible things. Annihilators. Adversaries. And a King of Shadows…who seeks a new soul to live in. The time of easy miracles is over. For Storm and Magneto, all the roads back to life are hard…and this is the hardest road of all."
Al Ewing lets it be known that the Dark Dimension that was taking over the White Hot Room when Phoenix was dying has many things Annihilation, Adversary, and Shadow King.
----- He gave us hints that these beings are all connected In X-Men Red 13. He let it be known that Annihilation had the power Storm faced before "Adversary" Storm faced Adversary in Uncanny X-Men and in Black Panther
----Al Ewing Stated in an interview that when Storm will be facing some of the X-men's greatest and most powerful foes over the years. Storm is allowed into The Mystery by the higher power of Marvel Cosmos but she is only allowed there to save 616 from cosmic threats,
-----Al Ewing confirms again they would be facing cosmic threats that will have a knock-off effect on the rest of Fall of X. Which is these cosmic entities and the dark cold room trying to take over the White Hot Room and the cosmos.
-----The writer of X-men Forever confirms he spoke with Al Ewing and issue 3 X-men Forever plays off on RoM
---When Resurrection of Magneto #3 came out Al Ewing stated he did research on various "oppositional forces to the Phoenix". He learned Shadow King was meant to be one of these by Claremont but has been aware of others that filled that spot like Goblin Force. He asked if were there any others and answered yes but did not get them all. Oppositional force to phoeni are plural but for the larger purpose of Fall of X, they should be coherently arranged. He stated Tiger God did not fit the oppositional force as he exists in the darkness around the WHR and the opposition force is in Tagiriron the opposite of the White Hot Room.
--- Playing on what's above Claremont created Adversary as universe primal chaos destroying the world and creating a new one out of chaos doing it over and over again. Adversary is literally beyond life and death meaning he can't die.
----Adversary shown in Howling Commandos of S.H.I.E.L.D. Vol 1 5 was stated to be the shadows themselves. As he was shown in X-Factor it was stated he was born in the "Darkness". Adversary was never from the physical plane and always needed a host. All Echo did was stop an incarnation he used.

It's been stated numerous times Adversary can not die and just him being locked up in a prison dimension for a long time by Roma would affect the universe. All Echo did was disperse his essence so he "couldn't" reform. This is also stated by Forge in X-Factor when using the spirit spell which was made to incap Adversary.
The writer of SoS directly stated that Devil Advocacy is from Journey into the Mystery and just shows Sebastian meeting the head demons in Marvel Multiverse. Adversary is alive with his peers as they still face off who should sit in Satan's chair.
Al Ewing confirms multiple times the oppositional forces to the Phoenix are the real deal in multiple interviews and his articles. He refers to them in one as cosmic threats. The next the most powerful foes of X-men over the years. He did make an article claiming to make a story about these oppositions. These entities are all linked by the darkness and have their own name/role but are aligned.
This as shown in Dark Cold Room/Shadow Kingdom when Storm pooled her force into SK combined with the rest of the entities... It was stated shadows can never be truly gone but can be robbed of their power for the time being. Showing that the oppositional forces of phoenix representing darkness can not be killed.