Rainbowstride is a pair of unique Conjurer Boots linked in many ways to fishing. First and foremost, it is a reward from Baited Expectations divination card and it is one of the available questlines from Krillson.
Rainbowstride is among the earliest confirmed fishing items, first released in v0.10.4b (2013/03/27).
Rainbowstride is a T3 drop-enabled Unique Item.
Rainbowstride is an outcome of the divination card Baited Expectations, which returns a Unique Fishing Item.
The Weeping Death prophecy causes Rainbowstride to drop from Dripping Dead. Dripping Dead is a native mob in Mud Flats, an area with another confirmed secret (the Albino Rhoa and its feather).
Cadiro Perandus has an unusually high chance of offering a Rainbowstride for exchange. This is much more common than the item's normal dropweight.
Rainbowstride is strangely absent from the Perandus Tackle Box, which only includes Slitherpinch, Fairgraves' Tricorne, an Albino Rhoa Feather, and a fishing rod.
Bringing Krillson a fishing rod works for completing Krillson's first-stage quest, but bringing Krillson a Rainbowstride does not work for completing Krillson's second-stage quest. We expect this may have to do with the specific wording of Krillson's second-stage quest: Krillson instructs you to "Fish up a pair of Rainbowstride boots", not merely obtain them.
Krillson's other dialogue contains a hint towards Fairgraves, which can lead players to discovering the secret interaction of Fairgraves' Tricorne enabling regular basetype fishing rod drops. Thus it is plausible that Krillson's many references to Rainbowstride may also be hinting at another as-yet-undiscovered secret interaction.
At the time Krillson was released to the game, many other Fishing Items did not exist yet. For example, the Writhing Jar was not yet released (though see the Caveats section, below). This puts special emphasis on Rainbowstride possibly being a critical clue.
The secret function of Rainbowstride, if any, cannot be accidentally stumbled upon. The item is very old, and was meta for some builds for a long time. Tons of players used this item during normal gameplay over the years, but none of that gameplay ever produced an unusual observation.
Thus "Equip Rainbowstrides and play the game, hoping to discover something" is an insufficient strategy. Rainbowstride's secret is much more difficult to determine than e.g. Fairgraves' Tricorne's secret (a player wearing Fairgraves' Tricorne will eventually accidentally discover a fishing rod, even if they do not understand how the drop occurred).
If Rainbowstride has an important secret, then:
Here is a list of the possible item condition variables that existed at the time of Krillson's release:
Esoteric manipulations of Rainbowstride might include:
In all things living, the light burns bright.
A passionate fire that consumes the heart.
The Spirit, it radiates not from above.
It is the warmth that draws you ever inward.
Where rest the truest colours of life.
Gold of the organ. Green of the throat. Red of the vein.
The myriad wet rainbow of the Spirit's domain.
In the 1.3.0c patch notes, a strange bug was fixed:
Fixed a bug where Rainbowstrides would incorrectly grant challenge completion for Sentari's Answer.
This refers to an old Challenge in Torment League, "Identify These Unique Items"
Identifying Rainbowstride was erroneously giving completion credit for Sentari's Answer.
(Screenshot of the bug in action: https://i.imgur.com/ZMnVwQY.png)
Sentari's Answer was released after Rainbowstride. Both items existed in tandem during Torment League. An indexing issue occurring later would be peculiar. This strange bug is most likely a silly parsing mistake, but it may also indicate something was hardcoded unusually on Rainbowstride that caused an odd edge case.
The lore quote on Rainbowstride refers to Gaius Sentari. Imagine a very naive implementation for checking challenge completion, such as:
When the player identifies a Unique Item, check that item's full database text for the string 'sentari'. If matched, grant Challenge Completion for Sentari's Answer.
Because the word "Sentari" incidentally appears in Rainbowstride's lore text, a silly implementation like this could produce this bug
Rainbowstride may not be the only Fishing Item important for Krillson's second-stage quest. Consider Slitherpinch. We know Slitherpinch has at least one secret function: it is one of the requirements for finding Angler's Plait. But there is evidence that Slitherpinch may have additional hidden functions. Slitherpinch was included in the Perandus Tackle Box (thus confirming it as a Fishing Item) long before Angler's Plait even existed.
The Weeping Death prophecy requires killing a rare Dripping Dead to obtain Rainbowstride. The prophecy text reads:
Striding dryly through the mud, yet dripping with rot, he waits for the Mother's touch.
You will track down a powerful Dripping Dead in a map who will drop a unique item when slain.
It is likely (but not guaranteed) that the capitalized word "Mother" refers to Hinekora, Mother of Death: the mob is undead, and the prophecy is spoken by Navali.
When this prophecy was first released in 2.3.0, it did not require the mob to be in a map. This allowed players to farm for a rare Dripping Dead in story locations.
Rainbowstride's basetype is Conjurer Boots which have Required Level 53. But at the time this prophecy was introduced, this meant Rainbowstride had a method of acquisition that did not require being in a zone that can drop Required Level 53 items. Thus even low-level characters were able to use this prophecy to obtain Rainbowstride.
Dripping Dead only spawns in these story locations: {Mud Flats, Fetid Pool, Flooded Depths, Submerged Passage}.
That means via this prophecy Rainbowstride is connected obliquely to Mud Flats twice: (1) Mud Flats contains the Dripping Dead mob; (2) the prophecy lore text refers to mud.
Rainbowstride was featured on the official site teasing Ascendancy, although this occurred after Krillson was released.
The subpage on which Rainbowstride appears is still accessible today. Rainbowstride appears in the section titled "Cruel Difficulty: Boots" here.
Strangely, Perandus Tackle Box does not include Rainbowstride, despite Rainbowstride existing since beta.
Timing Considerations of Krillson's Quests
It is reasonable to guess that Krillson's second-stage quest might have been completable as early as one of these dates:
You must be very close to visiting the Winding Pier. Once you have visited it you get to unlock it as the hideout.
However, we cannot be 100% certain that any of these dates is the true "breakpoint". In the past, GGG has indeed released inoperable content intending to tease future content. Here are two examples. (1) Vinktar Square: The Agnerod series were released bit by bit, but only after many patches did the staves finally serve a purpose in the vendor recipe that enabled players to access the Vinktar Square. (2) Prismatic Eclipse: This item was released and had modifiers based on white sockets before it was possible to place white sockets on it.
There are many more examples. Thus these dates are very reasonable assumptions, but not a guarantees.
Running with these assumptions is nice in that it hugely limits the number of combinations of interactions that could apply to Rainbowstride. But it also runs a risk of overlooking something. Use your judgment.
Finding a secret function of Rainbowstride would be a monumental discovery.
Keep in mind that any experiment you propose must be unusual in some way. Players have casually used Rainbowstride for huge amounts of person-hours. If a secret exists, it's not going to be easy.
Good luck!