In Deadfall, Ardethe was a planet in Kasterborous implied to be a ruined Gallifrey (specifically destroyed by the Daleks in the original AudioVisual story). Indeed it was later confirmed to be one of the Nine Gallifreys in @rassilon-imprimatur‘s The Story So Far...
Ardethe is home to the Shabooj'm, implied to be the descendants of Shobogan survivors.
According to All-Consuming Fire, many Shobogans on Gallifrey worshipped the Great Old One Azathoth.
The same book also confirmed that many of the powerful beings in the Whoniverse are Great Old Ones / Outer Gods from Lovecraftian lore going by different names (likely nerfed to varying degrees from their Mythos counterparts by the Anchoring of the Thread). The book features a creature claiming to be Azathoth, but the Seventh Doctor dismisses it as a fake, having little resemblance to its namesake.
One creature that does resemble Azathoth, however, is Akhaten. A slumbering idiot god, in the form of a (radioactively?) glowing large planet / brown dwarf. It is kept sleeping by an endless song, but it is foretold that it will destroy the universe when it wakes.
Azathoth was referred to as “the Nuclear Chaos”. This has been linked to radioactivity by later writers, but originally meant that Azathoth existed at the centre (’nucleus’) of the universe. Gallifrey has also been suggested to exist near the universe’s centre (ignore the fact that physicists don’t believe our universe has a centre).
Akhaten is worshipped by various species, including Merry Gejelh’s. Her unnamed species is humanoid, strongly-psychic and is seen wearing scarlet (with some beige-gold) in cultural ceremonies.
THEORY:
Is it possible that Merry’s species is in fact the Shabooj’m, or another distant descendant of the Shobogans that survived a destruction of Gallifrey, who went looking for Azathoth and found Akhaten?
They share some traits with Gallifreyans, being humanoid, psychic and wearing scarlet ceremonial robes (which could be especially relevant if one assumes that the Dalek-caused devestation of Ardethe is linked to the Time War, and thus possibly occurred after the dissolution of the chapters). Note especially the mix of beige-gold and red worn both by Merry and Ablif the Outsider in the images above. It’s definitely a stretch, but you could even make a link between the gold and white flowers Merry wears and the Gallifreyan Flower of Remembrance:
At least one species at Akhaten, possibly Merry’s, is also likely to be very long-lived, potentially with lifetimes similar to Gallifreyans. We know this thanks to The Doctor: His Lives and Times, which reveals that Susan bought the Doctor’s Five Hundred Year Diary as a present for him during their original visit to the system. Slow ageing and a Time Lord-like memory could also help to explain how Merry and other Queens of Years (a title which, when you think about it, is quite similar to ‘Lords of Time’) are able to learn every song and story of their culture.
The Akhet system is also, if Akhaten IS Azathoth, near the centre of the universe, placing it in the same region of space as Gallifrey and Ardethe, possibly within Kasterborous.
