The Citadel of Arelsis
Location: Mount Celestia / Lunia / Threshold
A dimension-bending castle perched on a cliff in an island out in the Silver Sea, the Citadel of Arelsis is home to the eponymous astral deva. Arelsis is tasked with detecting, capturing, and interrogating invaders of Lunia, preferably before they reach the beaches.
The chant goes that the island is still suffering from the after effects of a curse after it was hit by a meteorite. Where this unheavenly body came from and why it was cursed in the first place, well, that ain’t so clear.
Canonical Sources:
Tales of the Outer Planes (1e) p48-54 (‘An Element of Chaos’, an adventure exploring the immediate after-effects of the meteorite—it also explains what it was… but no spoilers here, cutter!)