How to create an artifact weapon linked to the progress of a mythic path?

Question

I want to pump up the group I'm GMing for with Mythic rules.
There is a warpriest in the group, and I wanted to have him earn the privilege of wielding the holy weapon of his church and make him a mythic champion.

My main problem is having this weapon to be powerful all along the campaign, without feeling like I gave him a +10 weapon at level 5. I also need that extra kick of potential to be accounted for in the mythic path. I have found this universal path ability:

Legendary Item (Ex)
You gain a legendary item. This item grants a number of abilities equal to your tier (maximum 3). At 3rd tier, you can select this ability again, increasing the maximum to six abilities and causing the item to become a lesser artifact. At 6th tier, you can select this ability again, increasing the maximum to 10 abilities; the item then becomes a greater artifact.

But it is very vague and I don't know the creation rules of this item.

However, I only own the core books, and I'm lost in the rules of d20pfsrd.

What are the relevant rules for creating such an artifact weapon?

Answer 1

Legendary Items are detailed here on d20pfsrd.

In summary: each legendary item has a number of abilities based on the Mythic Tier of the character who took the Legendary Item path ability and the number of times they've taken that ability.

Legendary Items also have Legendary Power, which allows the character wielding it (provided they're appropriately mythic) to use a Legendary Surge (which works a lot like a Mythic Surge, but is tied to something appropriate to the item - weapons might get the bonus to attack, eye-slot items to mental-stat-based skill checks, etc.).