Sentinel League 3.18 brought with it two new fishing-related Sentinels. The Infested Obsidian Stalker Sentinel and The Ancient Angler Emberstone Apex Sentinel.
When Sentinel launched, Fishcord found these two Mods in the data:
On Day 1 of Sentinel League, The Ancient Angler was still Unrevealed -- it had not yet been found by any player. We found flavour text that was unaccounted for among the other Revealed Unique Sentinels. The flavour text was:
"Text": "Under clouds of black beset by crimson lightning, we sailed stormy seas of poison and disease. Only one light shone in that darkness; only one Flame to keep hope alive."
With the word "Flame" capitalized, and the phrase "crimson lightning" (the Kalguuran description of Cataclysm), we expected this was referring to the Triskelion Flame; and with the sailing story, we figured it could be related to those fishing Mods.
One of Dannig's old saga dictations from Expedition League fit perfectly. In the voiceline Dannig_69.ogg, we found a nice match:
The lost journey across unruly and angry seas took nearly two seasons, during which we rarely saw the sky through the clouds of black beset by crimson lightning. We could not drink the rain, nor eat the fish, until the Flame cleansed the water and purified the meat. Even then, the sustenance was hollow. Our supplies were depleted and our men hungry by the time land emerged in the distance.
So the unaccounted-for flavour text was definitely referring to the Triskelion Flame. And Dannig specifies that it was used to cleanse water and purify fishmeat:
"cleansed the water" => sentinel_can_purify_corrupted_water "purified the meat" => sentinel_can_cleanse_corrupted_fish
A few hours after Sentinel league was launched, the first player found The Ancient Angler.
It is unknown at this time if there is any drop restrictions related to The Ancient Angler. The Fishcord is still collecting info on drops at this time.
While poorFishwife was doing casual testing, it was found that the Ancient Angler did not empower any rare or unique enemies.
Like other Apex Sentinels, The Ancient Angler says "Can only empower Rare or Unique enemies".
Contrast this to the special Apex Sentinels with restrictions, like The Angel of Vengeance. Rather than the line above, The Angel of Vengeance instead says: "Can only Empower Suncaller Asha (Level 83)".
So since The Ancient Angler has the usual line of other Apex Sentinels, and not a specially-constrained one like The Angel of Vengeance, I expected that The Ancient Angler would still empower enemies that meet the condition (like other Apex Sentinels do), plus do its other special effects (like other Apex Sentinels do). But that's not true.
A couple possible explanations (obviously not exhaustive):
If #2, then finding any enemy which does in fact get empowered by The Ancient Angler would be a very big discovery!
The best candidate location for observing something special might be the Alluring Pool, due to the Kalguuran lore referenced on The Ancient Angler. To my knowledge there are no other known fishing secrets tied to the Kalguur.