TLDR; Absolutely fantastic "one-shot" (I played it in two sessions, as the party took a turn I wasn't originally prepared for; nonetheless if you really want to flesh out the whole galactic level doomsday theme, i'd reccomend doing two 3-4 hour sessions). The material and storyline/hooks provided was more than enough to give the DM a good idea of a general direction to take the adventure. I had players at my table that have been playing since second edition and they said it was one of the funnest times they've ever had playing. If you've never DM'd for a tier 4 group I reccomend being really careful about the items you let the party have in the adventure. Personally, I loosely went off of what was alloted on pg. 135 in XGTE regarding distribution of magical items.

More in depth; In my party of 4 they EACH had: 1 Legendary, 1 very rare, 1 rare for equipment. For consumables, I alloted 3 VR, 5 each of R/UC/C. What made DMing difficult was the legendary items, of course. On top of them being literal hero's of their worlds. I just reccomend going over what they have and explaining you ultimately want them to have fun, but also don't want them to break the game. For example, our RP based character (which initially was able to blow past the young brass dragons based off RP alone) had a ring of three wishes. He had expressed he wouldn't do anything to break the game and I trusted he wouldn't. All he used it for was towards the end of the BBEG encounter was wishing the affects of the aura away and his second wish was for them to end up on the plane of Elysium when their time came. The first one saved them from a TPK and the second one just added more flavor to the adventure which is always good, even though the Planetar in the first chapter explicitly explains they'll all suffer the fate of death which he himself cannot escape. All in all, I did a lot of "rule bending" to keep the flow and to ensure everyone got their "moment". For those who have a RP based character, I reccomend using "Shinoh's" geas spell to turn the young brass dragon(s) against the party if they originally blew past them due to a high RP based roll. Normally I wouldn't do something like this, but it's a pretty big encounter that'll be missing and will drastically reduce the length of the one shot if they also choose not to lower the ward protecting the temple later on.

To the Author: Thank you very much for the fantastic story material, art work, and interesting character sheet for the elder evil/divine horror. This has inspired me to go on a journey of making a sort of consollidated homebrew "book" for creatures of the far realm as official sources don't go into great detail and are (intentially, unfortunately) ambiguous about the far realm in general.