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Deities have largely withdrawn their avatars from Faerun followingas a result of new rules imposed by Ao during the Second Sundering.

The question of exactly why the deities have unilaterally limited their use of avatars following the Second Sundering is as yet unannounced. Forgotten Realms creator Ed Greenwood confirmed that this is the situationappears to have been answered by Wizards of the Coast in their 2012 Gencon panel, but did not explain whyWhat is the Sundering (Part 1): The Overgod Ao established a new paradigm for the gods when dividing Abeir and Toril into separate worlds again:

The godsOnce it is over, the word of Ao declares that the Era of Upheaval is ended. Great stories remain to be told in general seem more "distant" post-Sunderingthis new era, more "heard from" than "personally seenbut they are not the stories of gods and godlike beings. They are the tales of mortal heroes, taking a stand to preserve the world they love.

AsThe first fifteen minutes or so of July 2018, whether a future official Forgotten Realms adventurethis video explain the lore behind this and what Ao's motivations are. The average person in the world has no knowledge of Ao's interactions with the gods, sourcebook oras stated in the "What is the Sundering" panel:

Elminster is probably the only one of these characters [of The Sundering series of novels] who might have any insight into that at all, and I don't think even he has very much.

This makes it unlikely that any given novel answerswill actually explain Ao's interaction, as it's something their protagonists are generally not aware of. Even sourcebooks do not have much information on this question remains to be seen. The Sword Coast Adventurer's Guide states that gods can still manifest on Toril, but only very rarely, and simply explains it as follows:

Though many tales are told of times past when the gods appeared in physical form and walked the land, occasions of that sort are few and far between.

Deities have largely withdrawn their avatars from Faerun following the Second Sundering.

The question of exactly why the deities have unilaterally limited their use of avatars following the Second Sundering is as yet unannounced. Forgotten Realms creator Ed Greenwood confirmed that this is the situation, but did not explain why:

The gods in general seem more "distant" post-Sundering, more "heard from" than "personally seen.

As of July 2018, whether a future official Forgotten Realms adventure, sourcebook or novel answers this question remains to be seen.

Deities have largely withdrawn their avatars from Faerun as a result of new rules imposed by Ao during the Second Sundering.

The question of exactly why the deities have unilaterally limited their use of avatars following the Second Sundering appears to have been answered by Wizards of the Coast in their 2012 Gencon panel, What is the Sundering (Part 1): The Overgod Ao established a new paradigm for the gods when dividing Abeir and Toril into separate worlds again:

Once it is over, the word of Ao declares that the Era of Upheaval is ended. Great stories remain to be told in this new era, but they are not the stories of gods and godlike beings. They are the tales of mortal heroes, taking a stand to preserve the world they love.

The first fifteen minutes or so of this video explain the lore behind this and what Ao's motivations are. The average person in the world has no knowledge of Ao's interactions with the gods, as stated in the "What is the Sundering" panel:

Elminster is probably the only one of these characters [of The Sundering series of novels] who might have any insight into that at all, and I don't think even he has very much.

This makes it unlikely that any given novel will actually explain Ao's interaction, as it's something their protagonists are generally not aware of. Even sourcebooks do not have much information on this. The Sword Coast Adventurer's Guide states that gods can still manifest on Toril, but only very rarely, and simply explains it as follows:

Though many tales are told of times past when the gods appeared in physical form and walked the land, occasions of that sort are few and far between.

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source | link

The question of exactly why the deities have unilaterally limited their use of avatars following the Second Sundering is as yet unannounced. Forgotten Realms creator Ed Greenwood confirmed that this is the situation, but did not explain why:

The gods in general seem more "distant" post-Sundering, more "heard from" than "personally seen.

As of July 2018, whether a future official Forgotten Realms adventure, sourcebook or novel answers this question remains to be seen.

The question of exactly why the deities have unilaterally limited their use of avatars following the Second Sundering is as yet unannounced. Forgotten Realms creator Ed Greenwood confirmed that this is the situation, but did not explain why:

The gods in general seem more "distant" post-Sundering, more "heard from" than "personally seen.

As of July 2018, whether a future official Forgotten Realms adventure, sourcebook or novel answers this question remains to be seen.

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Deities have largely withdrawn their avatars from Faerun following the Second Sundering.

The Forgotten Realms Wiki's entry on the Second Sundering (the events of the changeover between D&D's 4th and 5th editions) cites the Dragon Talk podcast of 15th December 2016, with designers Matt Sernett and Chris Perkins:

Gods have, to an extent, withdrawn from the world, ushering in a kind of 'Age of Mortals'. They are no longer speaking directly to most of their worshipers, instead sending signs and portents; e.g. In the Rise of Tiamat storyline, Tiamat's followers are doing all of the work to bring her onto the Material Plane, whereas before the Second Sundering, she likely would have sent an avatar to do some of the work.

In the particular case of Tiamat, it's not that she's forbidden by rule from personally entering the Material Plane, it's that deities are normally unable, which is why they must send avatars. According to AD&D 2nd edition's Faiths & Avatars, p.16, all deities above demigod rank are limited in this way:

Just as they can teleport across space without error, so too they can travel between the various planes of existence at will. These powerful beings cannot, however, travel to the Prime Material Plane.

However, she is forbidden by rule to travel to any dominion claimed by a pantheon which she is not a member of. By agreement, each pantheon of deities accepts the sovereignty of other pantheons over their dominions, according to Faiths & Avatars, p.4:

A pantheon holds ultimate sway within its own sphere of influence (if it is unconested) ... Disputes between pantheons and even between members of a particular pantheon are usually settled by meetings of the Circle of Powers in the Pavilion of Cynosure on a demiplane floating somewhere in the Ethereal Plane, which is held as neutral ground by all powers [deities] active in the Realms and provides an open forum for all parties involved in a dispute. A fundamental principle upheld by all pantheons active within the Realms is the essential sovereignty of a pantheon within its sphere of influence and, consequently, its right to act when threatened by agents of another sphere of influence.

If the gods still hold to this tradition, and Tiamat is currently considered part of the Faerunian pantheon, she may have every right to enter Faerun, and the other gods would be powerless to enforce any rule.

Another problem is that even if they do contest her presence, Tiamat's cult are not simply summoning her avatar: they're summoning Tiamat. A deity is substantially more powerful than an avatar, again, Faiths & Avatars:

When powers [deities] have vital business upon the Prime Material Plane, they must send avatars to act for them. An avatar is simply a manifestation of a deity upon the Prime Material Plane. This manifestation is not nearly as powerful as a power and is merely a projection of a deity's power to the Prime Material Plane. An almost infinitely vast gulf of power lies betwen the god and the avatar.

In other words, if Tiamat is successfully summoned, she will be significantly more powerful than any avatar those deities might send. Even so, we know that Tiamat, like all Faerunian deities, has been unable or unwilling to send their avatars to Faerun as commonly as was once done, which means that whatever force or situation stops her from simply sending avatars also imposes the same restriction in any deity who would send an avatar to try to stop her.