Please tell me I’m misunderstanding the following: Legacy Thistle Arrow=100+ bleed damage for 1d6 rounds.

Thistle Arrow, Greater Slaying Arrow, and Legacy Arrow

So normally, the suggestion for magic item creation that you can change the base weapon/armor/shield to a different kind, is just that, a suggestion. But the Legacy Arrow flat out states that it can be any kind of ammunition, not just ordinary arrows. So this means Legacy Thistle Arrows are a thing.

At 18th level and higher, Legacy Arrows can function as Greater Slaying Arrows, meaning if the enemy hit by it fails a DC 23 Fort save, they take 100 extra points of damage. And Thistle Arrows convert the damage they deal into bleed damage that lasts for 1d6 rounds.

Am I misunderstanding something here?

Well the bleed would be the regular arrow damage spread out over 1d6 rounds as a bleed, you're not gaining any damage.

The slaying is a secondary effect that would not be converted to bleed.

I don't believe you are but you are seriously over estimating how good it is. It's a once / week ability, that requires a super special magic item and being 18th level, still rolls to hit, dealing bleed damage if the opponent fails it's fortitude save at a medium difficulty.

Compare that to any of the good save or dies. It's just not that good.

That thistle arrow link goes to incomplete rules for Thistle Arrows. They do 1 point of bleed damage for 1d6 rounds and they don't cause that bleed damage to things immune to poison.

Full thistle arrow rules

That thistle arrow link goes to incomplete rules for Thistle Arrows. They do 1 point of bleed damage for 1d6 rounds and they don't cause that bleed damage to things immune to poison.

Full thistle arrow rules

Those are two different items named the same. If you looked, you would see they have different sources.

The more recent one is the one I linked.

In fact, your source is the D&D 3.5 Pathfinder rules, back when Pathfinder was just a 3rd party to D&D.

Slay Evil isn't the arrow's damage or a damage bonus on the arrow. It's a death effect attached to the arrow.

Thistle arrows don't "convert" damage to bleed, they just inflict bleed damage.

I actually like Greater Slaying Arrow at midlevels vs. undead/constructs, but by the time that Legacy Arrow ability comes online no one will ever fail the save.

The Nethys language is from a section in Adventurer’s Armory that is specifically intended to update the Pathfinder Chronicles item language from 3.5 to the Pathfinder rules. It appears unclear what it means; if it works as the OP suggests, i’d be much more concerned about random Inquisitors getting 100+ damage per arrow as he sprays everyone on the battlefield with standard 1 GP thistle arrows.

Thistle arrow rules:

3.5e Pathfinder Chronicles Campaign Setting version:

They deal normal damage but then become embedded in the wound and deal an additional 1 point of damage each round for 1d6 rounds from their irritating sap. Creatures immune to poison are immune to this extra damage. A creature can remove an embedded thistle arrow as a move action without provoking attacks of opportunity, but doing so deals an additional 1d3 points of damage as the thorny barbs are pulled free. A DC 12 Heal check (made as a standard action) can pull free a thistle arrow’s head without dealing any additional damage. A single thistle arrow costs 1 gp.

Pathfinder version:

This book updates several items from the Pathfinder Chronicles Campaign Setting

...and then it lists a number of items including...

Thistle Arrow

Crafted from the thistles of a poisonous plant, these barbed arrows deal damage as a bleed effect for 1d6 rounds after a hit.

So the latter is intended as an update to the former.

I believe what they intended to say is that it does 1 damage per round as a bleed effect for 1d6 rounds. But they neglected to say '1 damage'.

Without the context provided by the original version, the amount of damage is entirely unspecified, making it an invalid item.

As others mentioned, you need both sets of rules to get the correct effect for thistle arrows. The original effect was 1 point of damage for 1d6 rounds as poison, the updated effect is 1 point of damage for 1d6 rounds as bleed. Put simply, the update just changed them from a poison effect to a bleed effect, everything else is the same. Also, the bleed starts AFTER their initial normal damage, just as the poison effect originally did. Greater slaying alters the initial damage, not the bonus bleed effect of the arrows.