The text of the card:

Action: Choose an attachment with a printed cost of X in any player's discard pile and play that attachment for no cost. (The chosen attachment can belong to any sphere of influence.)

Does the last part mean that the attachment MUST belong to a sphere of influence? It doesn't seem to be a necessary addition otherwise, but it's parenthetical inclusion implies a clarification rather than an extra requirement.

Alas, no choice but to bring up the old Stand and Fight grievance… The wording of Reforged is quite similar to Stand and Fight, which was ruled/stealth-erratum'ed to not work with neutral allies. So, no neutral attachments.

FAQ reference: http://www.lotr-lcg-quest-companion.gamersdungeon.net/#Card51

I didn't think of that card or ruling, but yeah, it looks like the same form. Too bad. I really don't get the deliberate exclusion of neutral attachments. Does that make it OP or something?

Ironically, it means that if playing the saga you can't reforge Anduril.

Use it on neutral attachments, ignore the silly ruling.