Guild Wars 2

http://wiki.guildwars2.com/wiki/Ley-Line_Axe_Core

It says it’s part of a collection, which one?

If you look to the right on the page you linked, in the box, it states Collection: Machined Axe, right after Item Type, and before Rarity. The Wiki tells you which collection in those boxes for every item (for a collection).

“Only sold if you have the Machined Axe collection unlocked and only purchasable once per account.”

Says it right there.