Our current campaign is Tomb of Annihilation. We are playing it in the Forgotten Realms. As far as I can tell the adventure does not define a year in which it is supposed to play. We are playing it in Marpenoth of the year 1495 DR (about a month after our previous campaign ended, to be consistent with the fact that campaign was not subject to
the death curse, which started 20 days before the start of the adventure and affects everyone in the Forgotten Realms).
The characters have ties to the characters from that earlier campaign, which would make it difficult to set or recon it to be at a different year.
Now, in the adventure, several key events are described as having times relative to the adventure's time. The Spellplague is mentioned a few times, too, and I looked it up; that was supposed to happen from 1385 DR to 1395 DR. Unfortunately, this timeframe collides with some of the past events, and I want to understand if I can have these at that time during the Spellplague, or if I'd have to move them.
In particular, the following key event is affected
* "A little over a century ago, Acererak entered Omu and slew all nine trickster gods. He then enslaved the Omuans and forced them to carve out a tomb for their defeated gods." (p. 92, History of Omu). A little over a century ago would fall squat into the Spellplague timeframe. Acererak as a lich is an arcane spellcaster.
The Forgotten Realms Wiki quotes a fictious source that says
Magic spells faltered, or failed entirely. Countless spellcasters were killed or driven insane
but that it is written in an in-universe perspective which may be subjective. It also says
Despite efforts, with the structure of magic out of balance many spells failed [Richard Lee Byers (March 2008). Undead. (Wizards of the Coast), p. 69. ISBN 978-0-7869-4783-6] or produced unreliable results.[Bruce R. Cordell, Ed Greenwood, Chris Sims (August 2008). Forgotten Realms Campaign Guide. Edited by Jennifer Clarke Wilkes, et al. (Wizards of the Coast), p. 50. ISBN 978-0-7869-4924-3]
From a game perspective, would a powerful arcane spellcaster like a lich have retained enough of their powers and control during the Spellplague to enslave a city?