In the Year of Blue Fire (1385 DR), Cyric, with the aid of Shar, murdered Mystra, causing the Spellplague. We can read in The Grand History of the Realms (page 159) that
Tyr, Lathander, and Sune move against Cyric and successfully imprison
the Black Sun in his Supreme Throne, under a sentence of house arrest
to last one thousand years.
This is confirmed in the Forgotten Realms Campaign Guide (page 74) for D&D 4th edition:
Tyr, Lathander, and Sune united to imprison him in his nightmarish
playhouse of a plane, the Supreme Throne, where he remains to this
day, alone and increasingly insane.
I read to this day as the Year of the Ageless One, i.e. 1479 DR, as written in the same guide at the very beginning of the introduction via Elminster's words. The Sword Coast Adventurer's Guide sets the current year to 1489 DR; only 104 years has passed since Cyric was imprisoned. On page 27, there is only a little paragraph about Cyric and his church, without any details about his imprisonment.
I wonder what the consequences of Cyric's imprisonment on his church and worshippers are, since it seems there are none. Is Cyric actually free, and he can roam wherever he wants? Or is he still trapped in the Supreme Throne? Is his power diminished? Can he talk to / communicate with his worshippers?
Moreover, at page 45 of the Forgotten Realms Campaign Guide (among the examples for epic tier treasure) one can read
50,000 gp: A black iron key said to be one of the seven required to unlock Cyric’s shackles on the Supreme Throne.
Are there other references to these keys in official material (published books, manuals, adventures) or even tweets from Greenwood or other authors?
I am actually DMing a homebrew campaign and (theoretically) I can do whatever I want, but I prefer to be as faithful as possible to the official campaign setting.
If you have any details coming from current adventures (Princes of Apocalypse, Rise of Tiamat, etc.), please use the spoiler formatting.
Best Answer
Unless he is released, he is barred from direct involvement with anything outside his plane, (he should still be able to communicate, send messages, visions, and anything he could normally do from within his divine domain. He just can't leave it in any capacity.) His clerics might still gain spells from him, but he isn't present in the realm in any capacity, sealed in his own divine plane. When Gods and demi-gods are very real and able to directly involve themselves in the lives of their people, not being able to support and protect your followers can put severe strain on the God×Worshipper relationship. His influence may falter a bit, but gods are sustained by both the number and zeal of their devotees. Evil deities in particular might not have the same mass of followers as some of the good or neutral gods, but the tenacity and vigor of what few they have ( as they commit horrendously evil sacraments ) is enough for them to be on equal footing to the other greater powers.
Currently, his worship has diminished. If nobody worships or venerates him, he will become a vestige (dead power) and his petrified god-form will float in the astral plane until he is awakened (as a diminutive-god) or completely dissolved.