pathfinder – How to create an artifact weapon related to the progression of a mythical path?

I want to inflate the group for which I am Gming with Mythic rules.
There is a warrior in the group, and I wanted to earn him the privilege of wielding the sacred weapon of his church and making it a mythical champion.

My main problem is that this weapon is powerful throughout the campaign, without having the impression of having given it a weapon of +10 at level 5. I also need this extra potential to be taken in account in the mythical way. I found this universal path capability:

Legendary object (Ex)
You win a legendary item. This item grants a number of abilities equal to your level (maximum 3). At the 3rd level, you can select this ability again, increasing the maximum to six abilities and making the artifact a lesser artifact. At the 6th level, you can select this ability again, increasing the maximum to 10 abilities; the object then becomes a larger artifact.

But it is very vague and I do not know the rules for creating this article.

However, I only own basic books, and I am lost in the rules of d20pfsrd
Does anyone have any indications or know a good website?